Gemara

(Mishnah): If Reuven gave Shimon a gold Dinar and asked him to buy a cloak, and Shimon spent half for a cloak, and half for a Talis, both were Mo'el;

R. Yehudah says, Reuven was not Mo'el, for he could say "I wanted a big cloak, and you brought me a small, bad one."

The case is, Shimon bought a cloak worth a full gold Dinar with half the money. (If it was worth less, all would agree that Reuven was not Mo'el);

R. Yehudah holds that Reuven can say 'had you spent the full Dinar for a cloak, you would have gotten one worth two Dinars (or even more)!

Bava Kama 78b - Question (Rava): If Reuven said 'Alai Olah', and he separated an ox for this, and it was stolen, can the thief exempt himself by returning a lamb (according to Chachamim) or a bird (according to R. Eliezer ben Azaryah)?

(Mishnah): If one said "Alai (it is incumbent on me to bring an) Olah," he must bring a lamb (or a bigger animal);

R. Elazar ben Azaryah says, he may bring a Tor or Ben Yonah (a bird).

Since Reuven never specified what type of Olah he will bring, the thief may return any valid species;

Or, perhaps Reuven can say 'I intended to do an ideal Mitzvah (a big Korban. Therefore, you must return a big animal.)'

Answer (Rava): The thief can exempt himself by returning a lamb or bird, according to Chachamim and R. Eliezer ben Azaryah, respectively.

Bava Kama 116a: Rav Safra was in a caravan; a lion accompanied them. Each night, a member of the caravan would give his donkey to the lion, and the lion ate it.

On Rav Safra's night, the lion did not eat the donkey. Rav Safra made an acquisition on the donkey.

(Ravina): Really, no acquisition was needed. Rav Safra did so lest someone contest him.

Kesuvos 98a (Mishnah): If a widow's Kesuvah was 200 Dinarim and she sold property of the orphans that was worth 100 for 200, or property worth 200 for 100, in either case she received her full Kesuvah.

Question: When she sells land worth 200 for 100, the orphans tell her that she lost. When she sells land worth 100 for 200, why can't she say that she profited?

Answer (Rav Nachman): In our Mishnah, Rebbi taught us that the owner of the property receives the profits.

Rishonim

Rosh (Bava Kama 10:21, citing Rabbeinu Meir): If Reuven told Shimon to buy from a Nochri on credit, and he gave to Shimon to pay for him, and the Nochri forgot about the debt and he is not found, the money returns to Reuven. We learn from the case of Rav Safra. He gave his donkey only for the lion, but not for others. Also here, he gave the money for the Nochri, and not for the Shali'ach. Do not say that Ten k'Zechi (if one tells a Shali'ach 'give this to Ploni', when the Shali'ach takes it, he acquires for Ploni), so now that the Nochri forgot, Shimon acquires from Hefker. A Nochri does not have Zechiyah even mid'Rabanan (Bava Metzi'a 72a). Even if one told a Shali'ach to acquire for a Nochri, the Nochri does not acquire until he gets it. The money belonged to Reuven the entire time. How can Shimon acquire? Shimon cannot say 'I will take the money, for perhaps afterwards the Nochri will remember and claim it.' Since Shimon initially trusted Reuven (to pay for the merchandise), and became an Arev for him (the Nochri will claim from him if Reuven does not pay) without a security, also now he cannot keep the money for security due to the Safek (lest the Nochri claim). Shimon cannot say 'I want to give to the Nochri for Kidush Hashem', like the case of Shimon ben Shetach in the Yerushalmi. (He bought a donkey from a Yishmaeli, found a gem inside, and returned it for Kidush Hash-m.) One may do a Kidush Hash-m with his own money, but not with others'!

Rosh (ibid.): R. Efrayim was asked about such a case. He answered that if it would be clear that the Nochri forgot, the law is clear according to the opinion (in Kerisus 24a) that if one gave a gift and the recipient said 'I do not want it', whoever takes it acquires it. The giver cannot say 'I sent it with intent that he accept it. If not, it should return to me.' Rather, once it left his hand, he diverted his mind (despaired) from it. We say so about Edim Zomemim who testified that an ox must be stoned. When the owner heard the testimony, he made it Hefker (despaired from it), so whoever takes it acquires it. We say unlike this regarding Rav Safra's donkey! The Gemara explains that this is because Hash-m had mercy on him. Such miracles were not done for anyone else. The miracle was in order that Rav Safra keep his donkey. Forgetting is not a miracle. However, perhaps the Nochri did not forget. Reuven can say 'perhaps today or another day, he will claim from me.' Shimon cannot acquire for the Nochri, for a Nochri does not have Zechiyah. If Reuven is clever to claim this, the money returns to him.

Rebuttal (Rosh ibid.): In Kerisus, the case is that the recipient already took it. Therefore, if he later said 'I do not want it', he made it Hefker, for he already acquired it and the giver's intent was fulfilled. He was concerned only that the recipient accept it. He accepted it for a gift. The giver does not care if the recipient makes it Hefker afterwards. Here, the Nochri never acquired, so it returns to the sender. The Gemara (Kerisus 24b) said that R. Shimon ben Gamliel holds that one sends a gift with intent that the recipient accept it. If he does not accept it, it returns to the sender. There is no proof from Shor ha'Niskal. There, once the verdict was passed, it was Asur b'Hana'ah, therefore the owner diverted his mind from it. It is rare that witnesses are Huzam. Here, it was not Hefker. Even after he gave the money to Shimon, if he said 'give it to me. I will do business with it until the Nochri claims it', Shimon must give to him. Since Reuven still has rights in it, he did not divert his mind from it and make it Hefker at all. If they are given to the Nochri, fine. If not, they return to Reuven.

Mordechai (257, citing Maharam): This is like it says in Gitin 14a, if one sent money to Ploni, and he was not found, some say that it returns to the sender, and some say that it goes to Ploni's heirs. No one says that the Shali'ach keeps it!

Mordechai (Kesuvos 258): Reuven gave Shimon money to buy garments. He paid only part to the Nochri seller, and the Nochri forgot about the rest, and they could not find him. Two years passed. Reuven says 'give to me the rest. If the Nochri asks from you, I will pay him.' Shimon must do so. Above I ruled that the Shali'ach keeps the excess, i.e. when the Nochri erred and gave the Yisrael too much, and now we cannot find the Nochri. Then, the Shali'ach acquires the extra. Here, there is no reason why the Shali'ach should acquire.

Or Zaru'a (3 Bava Kama 457): A case occurred in which Ploni fled the city because he owed money to a Nochri extortionist. Later, he wanted to return. He sent four gold coins to Shimon to pay to the Nochri. Shimon compromised with the Nochri, and he took two coins and exempted Ploni. Ploni demands back the other two coins. R. Simchah learned from the case of Rav Safra. Here also, Ploni gave the money to divert the Nochri. He was able to do this for part of the money. Shamayim had mercy on him. How can the Shali'ach acquire the remainder?! Similarly, if a Shali'ach received a gold Dinar to buy a cloak, and he bought a cloak worth a Dinar for half the Dinar, and a Talis with the rest, Chachamim say that both were Mo'el, and R. Yehudah says that the Meshale'ach was not Mo'el. If the Shali'ach acquires the remainder, R. Yehudah would agree that the Meshale'ach was Mo'el! (In any case he gets only a cloak worth a Dinar. The Shali'ach did nothing wrong.) If a widow sold orphans' property worth 100 for 200, she received her full Kesuvah. She cannot say 'I was like a Shali'ach.' The Sugya of the stolen ox is according to R. Shimon, who holds that Davar ha'Gorem l'Mamon (something that one does not own, but it has value for him) is like one's property. Chachamim hold that the owner has no claim against the thief. Even R. Shimon holds that the owner can claim from the thief only the loss that he caused, which is only the value of a lamb or bird. It is not relevant here. If one said 'Alai Olah', and he gave to a Shali'ach enough money to buy an ox and bring it to the Kohen, and the Shali'ach bought a bird and brought it to the Kohen and he offered it, can the Shali'ach keep the rest?! The question was only when he separated an ox, for this is Gorem l'Mamon (causes a loss) only the value of a bird, and the thief does not have anything left from the ox. He pays only the loss he caused. The rest (of the loss) is of Hekdesh. The Torah exempted one who stole from Hekdesh. If the rest belonged to the owner, surely he must return it!

Poskim

Shulchan Aruch (CM 183:7): If a Shali'ach was sent to collect money from a Nochri, and the Nochri mistakenly gave extra, the Shali'ach keeps it all.

Shulchan Aruch (9): If Reuven owed money to a Nochri, and he gave to Shimon to pay for him and the Nochri forgot about the debt, he returns the money to Reuven.

Rema: Similarly, if Reuven gave 50 to Shimon to compromise with his creditors, and Shimon compromised for 25, Reuven keeps the remainder.

Beis Yosef (DH Kasuv): The Mordechai brought this in the name of R. Simchah. One must deliberate (whether the law is true).

Gra (28): We learn from Kesuvos. (The widow received more for the field than it was worth, and the orphans get the profit.)