Gemara

Rav Mari knew how to be Makneh to a Nochri, but onlookers, who did not know the proper way, would be Makneh in an invalid way.

Kidushin 8a (Rava): If one said 'be Mekudeshes to me for a Maneh', and he gave her a security, she is not Mekudeshes;

Since the Maneh is not here, the security takes no effect.

Bava Basra 157a - Question (Shmuel): If Yehudah gave Shimon a lien 'on property that I will acquire', what is the law?

Answer (Rav Chana - Mishnah): ...Levi's heirs say that Reuven died first, and the creditors say that Levi died first (so they can collect it).

If a lender cannot collect property that the borrower later bought and sold or bequeathed, even if Levi died first and Reuven inherited, they cannot collect from the heirs (who inherited from Reuven)!

157b - Rejection: The Mishnah is like R. Meir, who says that one can be Makneh Davar she'Lo Ba l'Olam (something that is not yet in the world), so surely this takes effect. The question was according to Chachamim, who say that one cannot do so.

Question: If you will say a lender can collect property that the borrower later bought and sold or bequeathed, if Yehudah borrowed from Shimon and then from Levi, and acquired property, who has a lien on it?

Answer: The Halachah is, both Shimon and Levi have liens.

Bava Metzi'a 78a (Mishnah): If Reuven hired Ploni, a donkey-driver (who then retracted), he hires others for more (at Ploni's expense).

(Rav Nachman): He may pay extra up to the wages (of what Ploni already worked).

Question (Rava - Beraisa): He may pay until 40 or 50 Zuz.

Answer (Rav Nachman): That is when the workers left their work Kelim with the employer.

Rishonim

Rambam (Hilchos Mechirah 11:17): If one stipulated with his wife that he will feed her daughter (from a previous marriage), he is liable, because he stipulated at the time of Nisu'in. This is like things acquired through words.

Rosh (1:2): R. Tam says that Rav Mari used to be Makneh the mother in order that the buyer acquire the ear of the fetus. People erred, and thought that he was Makneh (directly) the ear of the fetus, which is Davar she'Lo Ba l'Olam (it is not yet in the world; the buyer does not acquire).

Question: If a loan document does not say (that the lender has a lien on) 'what I will acquire', and the borrower acquired property and gave it through Matanas Shechiv me'Ra (when he was mortally sick), which Chachamim made like inheritance (Bava Basra 149b), does the lender collect from it?

Answer (Ramban, Teshuvah 38): No. Since he did not write 'what I will acquire', the lender does not collect from heirs at all. If one gave or sold what he will acquire, and accepted Acharayus on his property to uphold the property in the receiver's Reshus, he must do so. If not, the receiver collects due to Acharayus. This is if he wrote 'what I will acquire' in the Acharayus. Do not say that since there is no money, there is no security. One can accept a new obligation of Acharayus through saying 'I owe 100 to you in a document' (Kesuvos 101b). This is the Acharayus of all gifts, and all the more so through a Kinyan. However, if he sold them, perhaps the receiver cannot take from the buyer, since there is no limit. If the property was not sold, one always collects from the giver. One should be concerned for the matter, especially according to the opinion (the Rambam) that one cannot obligate himself in a matter without limit, for he does not know what he will acquire. Therefore, he is not Meshabed himself in a matter without limit, for the amount is not known. However, I disagree. (I hold that one can obligate himself in a matter without limit.)

Beis Yosef (CM 60 DH Kasav Ba'al, citing Sefer ha'Terumah): 'What I will acquire' always obligates. If he sold them, perhaps the receiver cannot take from the buyer, since there is no limit. However, if Kinyan Chalipin was done, we do not require a limit (Gitin 51a).

Mordechai (Kidushin 482): Since there is no money, the security does not acquire, since he was not Makneh the security itself. The same applies to other cases, e.g. if Reuven said to Shimon 'I give to you 100', and he gave a security for the 100. This has no effect. According to the letter of the law, he can retract and take back the security. R. Yehudah bar Yitzchak says that if one wants to give a security for a gift or Shiduchin, first he should acquire through a proper Kinyan the money and the Shibud (on the security - this is not in old texts of the Mordechai), and afterwards give the security, and the Shibud takes effect on it. Some say that 'since there is no money, the security does not acquire' applies only to Kinyan of a woman [or slaves], since they are acquired only through money, and the money he wants to give now is not intact. The security cannot be in place of it, since it will not remain with the receiver. However, if one gave a gift and left a security on it, he acquires. Meshichah on Metaltelim (the security) is in place of Meshichah of the gift. Bava Metzi'a 78a proves this. When the workers left their Kelim with the employer, he can use them to pay extra to other. Even though this is only for Devar ha'Avud (the employer loses due to their retraction), in any case (i.e. not Devar ha'Avud), he acquires up to their wages. I say that we do not distinguish. In every case, the security is not acquired. In Bava Metzi'a, he acquired the Kelim through a proper Kinyan. It was not a mere security. Sefer ha'Terumah says so. R. Baruch said in the name of R. Tam that it one gave a security on condition that he will give 100, the receiver may keep it until the giver gives. R. Yitzchak of Vienna says that we cannot learn from here to Shiduchin. However, we can learn that a security does not help for a gift. Rashi explained the case in Bava Metzi'a unlike R. Yehudah bar Yitzchak.

Poskim

Shulchan Aruch (CM 60:6): If one obligated himself in a Davar she'Lo Ba l'Olam, or he does not have it, is obligated. Even though one cannot be Makneh Davar she'Lo Ba l'Olam, this is when he uses an expression of sale or gift.

SMA (18): The Shulchan Aruch connotes that it depends on which expression he used. If one wrote 'I obligate myself to give to you...', even though it is a gift and the matter is not in his hand, the Chiyuv is on his body, which is in the world. Even according to those who say that if they acquired from him to give, or he said 'I will give', he did not acquire, for this is a mere verbal Kinyan, like the Rema (245:1) says in their names, here all agree that he acquired, for he obligated himself to give. The Rema below connotes like this. Regarding a document that says 'because I gave...', the Rema says 'because he was not Meshabed himself to be obligated.' This connotes that had he obligated himself, he would be obligated, even with an expression 'I gave', even according to the opinion that 'I gave this' is like 'I will give.'

Shulchan Aruch (ibid.): Even if one wrote a lien on a field, or accepted Acharayos on all his property (to collect from it what he obligates himself), and even if he gave a security, this has no effect.

Beis Yosef (DH u'Mah she'Chasav Aval and Bedek ha'Bayis: Sefer ha'Terumah brings from the Rif that a gift of Davar she'Lo Ba l'Olam does not acquire, even if he gave a security. He learns from Kidushin, in which we say that since there is no money, there is no security.

Darchei Moshe (6): The Mordechai brought from many Ge'onim that the receiver may keep the security until the giver gives. (Note: in our text, he brought this only from R. Baruch said in the name of R. Tam.) Also Hagahos Maimoniyos (Teshuvah 67 on Sefer Mishpatim) says so.

Shach (22): In the Shulchan Aruch, why did the Rema write simply like the Tur and Mechaber (that the giver may take back the security), and not say that some (the many Ge'onim) disagree?!

SMA (19): The Tur explains that whenever a Kinyan does not take effect without a security, the security does not help at all, unless he said 'acquire this amount in the security itself'.

Beis Yosef (DH Kasav Ba'al): Sefer ha'Terumah says that if one gave a gift 'from what he acquired and from what he will acquire', there is no Kinyan on what he will acquire. A Tosefta (Kesuvos 8:6) says like this. If one accepted Acharayus from what he will acquire to ensure that the receiver keep a gift, this has no effect. Since the gift does not take effect, the Shibud and Acharayus do not take effect, like we say in Kidushin (8b). The Rif said so in a Teshuvah. However, if one gave land with Acharayus, and he was Meshabed property that he acquired and what he will acquire, to remove all claims against the field he gave, all agree that in every case of Shibud, 'what I will acquire' helps, and we take from people who bought from him.

Shach (21): Gidulei Terumah asked that this is unlike the Ramban that the Beis Yosef brought, that if he accepted Acharayus, he is liable. I say that the Ramban discusses one who obligated himself and accepted Acharayus to uphold the properly in the receiver's Reshus. Therefore, even though the Kinyan is void, the Chiyuv takes effect on his body to uphold the gift. This is clear from Sefer ha'Terumah. If not, why did he say that Acharayus does not help, and immediately cite the Ramban (if the Rambam disagrees)? Sefer ha'Terumah says like I said in Sha'ar 64:2, and says that the Rif said so in a Teshuvah.

Shulchan Aruch (ibid.): However, if he used an expression of Chiyuv, e.g. 'be witnesses against me that I obligate myself to pay this sum to Ploni', he is liable. This is if a Kinyan (Chalipin) was done.

Beis Yosef (DH veha'Mechayev): We hold that one can be Meshabed (put a lien on) what he will acquire. Sefer ha'Terumah learns from this that one can obligate himself in a Davar she'Lo Ba l'Olam.

Beis Yosef (DH u'Mah she'Chasav Ela): Saying 'be witnesses against me' is no worse than an Arev (guarantor), who obligates himself through a Kinyan, even after the loan was given.

Shach (25): He is liable to pay only from Bnei Chorin (property he still has), but not from what he sold, even though he wrote a document. A document does not help, even if the amount is limited, e.g. this house or sum that I will inherit. Since it was not in the world (i.e. his possession) at the time the document was written, there is no publicity. Rashi and Tosfos (Gitin 50b DH l'Fi) hold like this. However, the Rambam (brought above) disagrees.