Gemara

A case occurred in which Reuven rented a mill to Shimon. They agreed that in lieu of paying, Shimon would grind Reuven's grain. Reuven waxed rich, and bought a mill and a donkey to grind.

Reuven: From now, [I do not need you to grind for me, so] pay me money.

Shimon: No, I will continue to grind for you.

(Ravina): This is like our Mishnah. The two men [obligated to feed a girl] cannot say 'we will feed her together.' Rather, one feeds her, and the other gives to her the monetary equivalent.

Rejection (Rav Avira): There, a girl has no use for two portions of food. Here, Shimon can tell Reuven "use your grinder to grind and sell to others, and I will grind for you!"

This is only if Shimon cannot find people who will pay him to grind for them. If he could find such people, Kofin Al Midas Sedom. (If one can help another without losing himself, we force him to do so.)

Bava Kama 104a: If Reuven stole from Shimon and admitted to him and could return the money, it is as if Shimon said 'the stolen object is like a deposit with you';

If Reuven swore, he must travel to return the stolen object in order to [bring his Asham and] get atonement.

Nedarim 55b (Beraisa - R. Yehudah): If one was suffering because he was wearing wool, and vowed 'wool will not come upon me', he may not wear it, but he may carry it;

If he was carrying flax and sweating, and said 'flax will not come upon me', he may wear it, but he may not carry it.

Rishonim

Teshuvas ha'Rashba (793): If Reuven swore to pay Shimon on a certain day, and when the day arrived Shimon is not there, Reuven is exempt until Shimon comes. He need not pay Shimon's wife or children. Even if he paid them, he is not exempted [of his obligation], for this is not payment. Only a thief who swore [to deny the theft] must bring the money to its owner, even if he is in Madai (overseas). If he did not swear, it is like a deposit in his hand until the owner comes for it. We are no more stringent about paying a lender. He need not make a Beis Din to hand the money over to them. If he did, this has no effect.

Question: Reuven swore to pay his debt to Shimon in Shimon's city Ploni, on a certain day. Before the day arrived, a known Ones occurred, and Shimon needed to move to city Almoni. Must Reuven take the money to Almoni?

Answer (Maharik 112): If Almoni if further from Reuven than Ploni was, surely he need not go to there. If it is closer, it seems that he must go there to pay. Even though his oath mentioned only Ploni, Avi ha'Ezri rules that Kofin Al Midas Sedom. The Mordechai says so in Bava Kama (16). We even force a Ba'al ha'Bayis to lend Chatzeros to another, even though he did not stipulate at all, and had no obligation. Here, he was obligated, and stipulated [to pay on the day], and the lender did a favor to him. Even the Ri, who rules that Ein Kofin Al Midas Sedom, would agree in such a case. In several places in the Gemara, we say "in a case like this, Kofin Al Midas Sedom", even when there is toil, and he might come to a loss. If Shimon can find people who will pay him to grind for them, we force him, even though it is not certain that he will find. "In a case like this" shows that we may learn to other cases even if they are only somewhat similar.

Poskim

Shulchan Aruch (CM 73:8): If Reuven swore to pay Shimon on a certain day, and when the day arrived Shimon is not there, Reuven is exempt until Shimon or his Shali'ach comes. He must have the money in his hand on the day, and he may not spend it.

Beis Yosef (DH Kasav): Since the lender is not here, and he did not claim from him, it is as if he said 'hold onto it until I come.' In another Teshuvah (attributed to the Ramban, 247), the Rashba wrote that on the day Reuven swore to pay, he must have the money in his hand, in case Shimon or his Shali'ach will come now. Presumably, he may not spend the money, so it will be ready for payment when he comes.

SMA (23) and Shach (26): Even if Shimon is in the city, Reuven is liable for transgressing his oath only if Shimon demands payment [and Reuven does not pay], and all the more so if he is not in the city.

Shach (24): He need not give it to his wife or children, for this does not exempt him. Based on this, if Shimon's wife does business in his house, Reuven must pay her, according to the opinion (Rema 120:2) that this exempts him.

Gra (25): He cannot make a Beis Din [and leave the money in Beis Din], for all agree that giving not in front of the recipient is not called giving (Gitin 14a).

Rema: He is exempt when Shimon is not there only if he did not live in the place he lived at the time of the oath, or he left. If he lived in the place he lived at the time of the oath, Reuven must take the money to him, for he initially swore with this intent.

SMA (26) and Shach (25): "He left" means that he did not change residence, just he left for business.

Gra (27): Vows depend on his intent, like it says in Nedarim 55b and 63b.

R. Akiva Eiger (cited in Pischei Teshuvah 13): Even without an oath, this is the law.

Rema: Some say that even if he changed his residence, if he is no further from Reuven than initially, he must bring the money to him.

SMA (27): This does not argue with the previous opinion, just there is a Chidush.

Shach (27): One can reject all Maharik's proofs for this law. We hold (Shevu'os 26b) that one is exempt for an oath if he did not say it with his lips. If so, he is liable only for what he swore, i.e. to take the money to Ploni. He need not take it anywhere else, even if it is no farther. Had he not sworn, he would be exempt, therefore he is liable only for what he said. Maharik himself asked this! He answered "this (one is liable only for what he said) is only for one who forbids to himself. We do not learn from there to a similar case, even if he clearly intended also for the other case. It says "Levatei bi'Sefasyim." When one swears to fulfill a Tenai based on another's request, it depends on the Tenai. The oath takes effect on whatever the Tenai applies to. The Rosh (Teshuvah 8:6) connotes like this."

Rebuttal (Shach): It is unreasonable to distinguish like this. Also, obviously one who swore by himself to pay is liable only due to Levatei bi'Sefasyim. This is Shevu'as Bituy, so it is only for what he said. The Rivash (195) and Rema (YD 218:1) connote like this regarding one who was in pain due to wearing wool garments, and swore that wool will never come on him. He intended only for wearing garments, but he may carry wool. They wrote that this is only for one who vows to himself. If one vows to another, we follow whatever the words of the vow include, unless there is a clear Umdena (assessment of intent) otherwise. This shows that [also] regarding one who vows to another, we follow the words of the vow. We follow a clear Umdena, but only to exempt him, for surely he did not intend for this. We do not say oppositely to obligate him what he did not say.

Shach: Maharik learned from Teshuvas ha'Rosh. The Rosh said "if one swore to pay within a certain time, and the lender did not claim within the time, he must pay afterwards due to the oath. This is unlike one who swore to eat a loaf today, and did not eat it. He transgressed his oath, and need not eat it later. His oath was primarily about eating it that day. Here, he must pay, and he swore to pay. His oath was primarily about paying the debt. The time makes him zealous to pay, just like one who lends for a set time. If he does not pay at the time, he must pay later." His words prove that his oath was primarily about paying. As long as he did not pay, there is an oath. I see no proof that he must go to Almoni (where the lender is now), in particular if the borrower has an excuse, e.g. he had to go anyway there (to Ploni, the place he swore about), but he has no need to go to Almoni. Alternatively, "I used to go to Ploni, or I could easily persuade someone to take me there for money, or I knew someone I trust who used to go to there (I intended to send the money with him), but I do not know someone I trust who goes to Almoni," or similar excuses.

Gra (28): Only if there is a great proof we say that he did not intend to swear about [a similar case]. We do not say so in a case like here. One who was wearing wool... may carry wool, but he may not wear anything [of wool], even if it pains less than the first (what he was wearing when he swore). If he was carrying flax, he may not carry any flax, even a load lighter than the first. All the more so, this applies to one who swears to another person.