A lady brought me her almost-new sheitel to style and set. When she came to pick it up, it was not in the place where we keep fixed sheitels. I told her I would look for it and she should come back in a few days. We looked all over and could not find it. Therefore, when she came back I paid her for her sheitel. A few weeks later a lady came and brought me the sheitel. Apparently what happened was that one of my workers by mistake gave her the sheitel thinking it was hers when actually it belonged to someone else. I called the first lady and told her that her sheitel was found and she can have it back and should return the money I paid her. She replied that she is happy to get back her original sheitel but she used the money that I gave her to buy a new sheitel which she would give me. I replied that since I gave her money she must return money and not a sheitel which she had worn. She replied that she did not need, and could not afford, two expensive Shabbos sheitels. Can I force her to return money or not?
The first question we must address is who is the owner of the original sheitel.
The Mishna (BM 33B) discusses a person who was given an animal or object to watch and the animal or object was lost or stolen. The Mishna rules that if the watcher paid for the object willingly and then the thief was found, the double amount the thief pays (since he originally denied and swore to the effect that he did not steal the object) goes to the watcher and not to the owner. The Gemoro explains that the reason is because at the time when the owner originally gave the object to the watcher, the parties made a tacit agreement that if the object gets lost or stolen and the watcher willingly pays for the lost or stolen object, the owner gives the watcher the object right away in exchange for the money which he eventually pays. That is why the doubled amount goes to the watcher and not to the owner because, retroactively, the animal or object actually belonged to the watcher at the time it was stolen and not to the original owner since the watcher willingly paid for the stolen object after it was stolen.
This would seem to determine the answer in your situation since you had the status of a watcher when the sheitel was by you since the Mishna (BM 80B) rules that any craftsman has the status of a paid watcher (shomer sochor) over the object he is paid to work on. Since you readily paid for the lost sheitel, according to this approach you acquired the old sheitel. She can keep the money (or new sheitel which she bought) and you have no claim whatsoever. This seems to be the opinion of Tosafos (BM 33B kegon) and is the ruling of the Shach (CM 295, 11). According to this approach your question is answered: you own the sheitel she brought you to style and she can keep the money you paid her (and thus the new sheitel).
However, the Rambam (Sheilo 8, 1) rules explicitly that in the case of the Gemoro that we described, if the object is found it reverts to its original owner. The Shach finds this very difficult because the simple understanding of the Gemoro is like Tosafos that a watcher who pays for a lost object thereby acquires the lost object and the Shach goes so far as to say that the Rambam is only speaking about a very special case but in general he too maintains that the watcher acquired the lost object and the Shach rules that way, as we noted above.
However, many Acharonim including the Ketsos, Nesivos, Reb Chaim (Mechiro 23), Ohr Someach and more, disagree with the Shach and understand that the Rambam is talking about all cases and maintains that whenever the object is found it belongs to its original owner in spite of the fact that the watcher paid for it. Some (e. g. the Ketsos) even maintain that Tosafos also agrees with this Rambam and there are many approaches to resolve the fact that the Gemoro seems to disagree with the Rambam. Moreover, there are other Rishonim (Kesef Mishne ibid cites the Talmidei Horashbo, and there is a Tosafos Rid (BM ibid)) who agree with the Rambam and deduce his ruling from other sections of the Gemara. (Ohr Someach (ibid) cites BK 57B as a source for the Rambam.)
There are opinions (Ulam Hamishpot 295) that attribute the Rambam’s ruling to the fact that in retrospect the watcher’s payment was made in error since he only paid because he could not find the object. Since it was paid by mistake we reverse the payment. However, many others (including Shach 295, 6 and Rashash on BM 35A) disagree on this point and maintain that this is irrelevant.
Having established that there is a dispute and that the Rambam maintains that you can return the customer’s sheitel to her, we have to study whether, according to the Rambam, you can demand that she return your money or your customer can give you the sheitel that she purchased with the money you gave her.
Since this question is not discussed by the Gemara and Rishonim, and we must derive the law from an analogous situation, to do this we have to analyze the situation at hand, according to the Rambam’s approach.
According to the Rambam the payment you made for the lost sheitel was necessary at the time but, given the new situation where the customer’s sheitel was recovered and may be given back to the customer, the payment was unnecessary and needs to be reversed. A similar situation exists when a person buys an object which is later discovered to be defective. Since the object was defective, the customer is entitled to reverse the sale. In order to reverse the sale, the customer must return the defective item in order to force the seller to refund his money. This is similar to your situation where your customer must return the payment she received from you in order to recover her sheitel. Your question boils down to a question whether returning the sheitel that she purchased with the money you gave her, constitutes fulfillment of the condition that she must return the money which you gave her.
While this question is not discussed in the context of a sale that is reversed because the lost object was found, it is discussed in the context of a sale where the object was discovered to have a defect.
The Mishna (BB 92A) discusses a person who bought seeds with the understanding that he will plant the seeds (and not consume them). If he plants the seeds and they do not grow, the Mishna rules that the customer is entitled to a refund of his money even though he has no seeds to return. The Ritva explains that the rationale is that since, when the customer bought the seeds it was with the clear understanding that he would plant them, it is as if the seller told him to plant them. Therefore, the customer is certainly not liable for the loss of the seeds and it is as if he returned the seeds since he planted them as per the seller’s instructions.
The Ritva generalizes that this is true whenever a sale is reversed because it is a mekach to’us; if the customer did what was expected he is not penalized for having altered the article he purchased. As an example, he mentions one who bought a garment and altered it according to his needs. He says that the customer may return the altered garment, a ruling that is cited by the SA (CM 232, 14) without dispute.
The Pischei Choshen applies this rule to one who purchased a defective sefer. If a customer bought a sefer and wrote his name in the sefer, he still may return the sefer if he later discovered a defect, and he may demand a refund of his entire payment since it is normal for a person to write his name in a sefer that he purchases.
Similarly, in your situation, according to the Rambam’s approach, the transaction where you paid money in return for the original sheitel needs to be reversed just like it is reversed when one discovers a defect in the object he purchased. In your case, when you paid the customer it was expected that the customer would perhaps, use the money to purchase a new sheitel. Therefore, we can derive from the law concerning sales of imperfect objects that according to the Rambam your customer can present you with the sheitel she bought and demand the return of her original sheitel.
In conclusion: In any case you cannot force the customer to return money. The only issue is whether you keep the original sheitel (Shach) or the new sheitel (Rambam).