Gemara

(R. Chiya bar Aba, and a Beraisa): If Reuven admitted to Shimon that he owes him 100, and tomorrow he says 'I was only joking', he is exempt.

Bava Metzi'a 30b (Mishnah): If Reuven lost one Sela of earnings while returning an Aveidah, he may not demand full compensation. He is paid like a worker.

If he wants full compensation, if there is a Beis Din there, he stipulates with Beis Din; if not, he is exempt from returning, because he would lose money.

Kidushin 8b - Question (Rav Mari): If a dog was chasing Leah, and David said 'be Mekudeshes to me with a loaf, and she said 'give it to the dog', what is the law?

With the Hana'ah that she is being saved, she is Makneh herself (to him).

Or, can she say 'the Torah obligated you to save me!'

This question is unresolved.

(Beraisa): If one said 'be Mekudeshes to me with a loaf', and she said 'give it to this Oni', she is not Mekudeshes, even if the Oni relied on her for his food.

This is because she can say 'also you must feed Aniyim.'

Yevamos 106b: Rav Papa's wife's sister fell to an unworthy Yavam. Rav Papa told him to do Chalitzah on condition that she will pay him 200 Zuz. Afterwards, Abaye said that she should pay him.

Rav Papa: She was just fooling him!

(Beraisa): If a man was fleeing jail, and offered the ferryman a Dinar (a large amount) to take him across, he owes only the usual fare.

He can say that he was just joking. Also she can say that she was just joking!

Bava Basra 141b (Beraisa): If one said 'the first person who will tell me that my wife gave birth to a male, I will give to him 100', if she gave birth to a male, the informer receives 100.

Rishonim

The Rif and Rosh (Bava Metzi'a 16b and 2:23) bring the Mishnah.

Rambam (Hilchos Gezeilah 12:4): If Levi found an Aveidah and stipulated with the owner or in front of Beis Din to be compensated for what he will lose from his work if he takes the Aveidah to return it, he receives it. If the owner or Beis Din is not there, his own (wealth) comes first (he is exempt from returning, lest he lose).

Rambam (7): If one was fleeing jail, and offered the ferryman a Dinar to take him across, he owes only the usual fare. If one asked a fisherman 'cease fishing and take me across', he must pay whatever he stipulated.

Rosh (Yevamos 12:15): Rashi says that Abaye held that the Yevamah must fulfill her Tanai. This is wrong. When one says 'on condition that', he need not fulfill his Tanai. He may choose not to, and the Ma'aseh will be Batel! If one gave a Get 'on condition that you will give to me 200 Zuz', she is divorced only if he willingly took the money. He need not accept it (Gitin 74a); payment against the lender's will is invalid (Bava Metzi'a 77b). Rather, since Chalitzah cannot be Batel, she must pay what she promised, like one who hired a worker. Rav Papa answered that source he was not proper for her, he was obligated to do Chalitzah. All that she promised was more than is proper. This was like Ones, like the fleeing prisoner.

Rivash (476): One must fulfill everything he promises to give for wages, even if there was no Kinyan. Abaye held that the Yevamah must pay what she promised, like one who hired a worker. Rav Papa answered that since he was not proper for her, he was obliged to do Chalitzah. All that she promised was more than is proper. This was like Ones, like the fleeing prisoner.

Mordechai (Sanhedrin 704): If one admitted to Levi that he owes him, we do not suggest that he was joking, for it is unusual to joke about this. We do suggest this regarding a fleeing prisoner, an unworthy Yavam, or one who told his rich son-in-law 'learn with your son, and I will pay you.' This is because his son-in-law is not poor, and in any case he must teach his son.

Poskim

Rema (CM 81:1): If Reuven told his rich son-in-law (David) 'learn with your son, and I will pay you' he is exempt, since David is not poor, and in any case he must teach his son. Therefore, Reuven can say 'I was joking',

Shach (5): Even if David is not rich, since he is not poor he must teach his son or hire a teacher. This is why later (131:1) the Rema omits the word 'rich.'

Rebuttal (SMA 5): Also in Siman 131, the text of the Rema should say 'rich.'

SMA (6): Normally, one can say 'I was joking' only if a claim was made against him, but not if he admitted by himself. Here is different, for there is reason to say that he was joking, since David is obligated anyway.

Rebuttal (Shach 6): We require saying 'I was joking' only if one admitted that he already owed. One who promises to pay is not liable, for in any case David needed to teach his son. This is also why the ferryman gets only his proper wage, and the Yavam does not get anything.

Shulchan Aruch (265:1): If Levi found an Aveidah and stipulated with the owner or with Beis Din to be compensated for his loss, he receives it.

SMA (5,6): This refers to all the wages he lost. Chachamim taught that Stam workers would accept less than this to be idle and not need to toil; he revealed that he needs to toil to earn much to support his family.

SMA (6): He does not transgress taking wages for Hashavas Aveidah, for one is exempt when there would be a loss.

Rema: Levi is obligated to return, because the owner wants to pay his loss and is obligated to give all that he stipulated, even if he stipulated more than is proper.

SMA (7): I.e. he stipulated to get more than Stam workers, who accept less than their full wage on condition to be idle. If he stipulated to receive more than his full wage, surely one need not pay this. One can say I was joking even regarding Reshus, all the more so when Levi has a Mitzvah to return it.

Shulchan Aruch (YD 336:2): A doctor may not take wages for his Chachmah and Limud (teaching). He may take for his toil.

Gra (11): This is unlike the opinion that the prisoner need not honor his stipulation because the ferryman was obligated to save him due to Hashavas Aveidah. If so, the same applies to a stipulation with a doctor. This is wrong, for we brought a proof about Chalitzah from a prisoner. The Yavam had no obligation! Rather, one need not honor a stipulation made amidst Ones and duress. Even though the Yavam was not obligated to do Chalitzah, and (by not doing Yibum) he lost a wife and inheritance (of his brother), the Torah suggested giving proper counsel (Yibum is improper). However, Rashi says that she was exempt because the Torah teaches that Yibum is not always proper (and in this case, it was not). In Kidushin we asked whether a woman who accepted Kidushin to be saved (from a dog) can say 'the Torah obligated you to save me!' We did not learn from one who told a man (who asked to be Mekadesh her) 'give it the Oni.' The Ran says that perhaps when her life is saved she resolves to be Mekadesh herself.

Binyamin Ze'ev (280): A case occurred in which Ploni offered a doctor 50 if he will cure his son, and afterwards wanted to pay only his proper wage. The doctor receives 50. Semag (Aseh 74, 153a) compares this to the fleeing prisoner. He must pay like he stipulated. It is normal to pay high prices for Refu'os; he cannot say 'I was joking.' He is obligated not only because the doctor pained himself. Rather, he must pay the wage for the doctor telling him what to do, like it says in Bava Basra 141b. How did the one who informed him acquire the money? This is like we say in Bava Metzi'a 69b, that he gets paid for saying. Also R. Simchah in Or Zaru'a says so; we find that words are sold.