Question
On my last trip to the United States, an Israeli friend asked me to purchase a computer for him in the United States. I complied with his request and bought him the computer. I intended to charge him for this service but did not tell that to my friend. May I later charge by either asking for the money or just add the amount I wish to earn onto the amount I paid, and just ask for the entire amount? Also, when I returned, I found out that my friend didn't own any dollars at all. Does this pose a problem?
Answer
When one purchases something on someone else's behalf there are two approaches the agent can take. The agent can act as a shliach for the one who asked him to purchase on his behalf or he can buy it for himself and then, upon his return, sell it to the one who sent him.
There are various repercussions from each approach. For example, in your situation, if you acted as an agent to buy the computer on behalf of the one who sent you, as soon as you bought the computer, the one who sent you became the owner of the computer and he owed you the money that you spent on the computer. Since the computer belonged to the one who sent you, if something happened to the computer on the way back, you would have the status of a shomeir chinom on his computer and would only be liable if you acted carelessly.
However, if you bought the computer for yourself then it was yours. One result is that if something happened to the computer on the way back you would bear the loss. Additionally, if the one who asked you to buy the computer refused to accept the computer you would be stuck with the computer.
The SA (183, 4) discusses your case and rules that if A asked B to buy something on his behalf and B bought the object as requested, A became the owner of the object and B cannot later claim that he bought the object for himself. The reason is because we assume that he acted in response to A's request. The Rishonim derive this from the Mishna (BM 9B) that rules that if A, who was riding on an animal, asked B to pick up an ownerless object on his behalf, and B picked it up and gave it to A, B cannot afterwards claim that he picked it up for himself.
Thus, we see that if A asks B to buy something on his behalf, A's intent to become the owner of the object when the B purchases the object. Therefore, when you bought the computer for your friend, your friend became the owner of the computer.
You also asked whether the fact that your friend doesn't own any dollars poses a problem. Since you bought the computer on behalf of your friend, when you paid for the computer, you lent money to your friend. Since you paid with dollars, the loan automatically is a dollar loan (See Chavos Da'as 162, 1) unless you made up otherwise. Since dollars are not legal tender in Israel and YD (162, 1) states explicitly that only legal tender is not an object, the dollars are an object and not money.
The law is that one may only lend an object if either the object has a fixed price or if the borrower owns some of the object (See YD 162, 2). Since you lent dollars and the price fluctuates constantly, one cannot say there is a fixed price. Therefore, the only reason to permit the loan is if the borrower has some of the item he borrowed. Since the one who sent you did not own dollars you unknowingly lent in violation of the laws of ribbis.
Since there is a prohibition to lend objects unless one of these conditions is fulfilled, the commentaries (Bach, Taz (162, 3)), rule that one is not allowed to lend an object before ascertaining that the borrower has some of the object that he is borrowing. Thus, it was your responsibility to ask your friend if he owned dollars before buying an object for him that is sold for dollars. You cannot claim now that you did not intend to buy for your friend because you would not have wanted to violate a ribbis prohibition, since it was your responsibility to check this out beforehand.
Since you lent dollars in a manner that was forbidden (see YD162, 5), your friend is only obligated to pay you the lesser of the shekel value of the dollars on the day you bought the computer and the value on the day he pays since you are not allowed to gain from the price changes of the dollar. Since the dollar was only worth x amount at the time you bought (which is the time you lent), if the dollars increases in value to y, the increment between x and y would be ribbis. However, if the price of the dollar drops and y is less than x you will only be able to charge y.
You also ask whether you can charge for purchasing on your friend's behalf.
The Rosh (64, 3) was asked whether a person who was told that he would be paid to sign as a cosigner on a loan can afterwards claim the amount that he was promised. The Rosh ruled that even though the cosigner was told he would be paid, the borrower is not obligated to pay since people then did not pay cosigners.
He compares this to one who promised to pay a large amount of money to someone to help him escape from jail on a boat. The Gemara rules that he is not obligated to pay any more than the usual salary of a boat owner.
Thus, we see that even if one promised a fee to someone for doing something which people usually do for free, he is not obligated to pay. Certainly, here where no one promised you a salary and people usually do not charge for this favor, you may not charge for your service. You also may not add the fee that you wish to earn onto the price of the computer without telling your friend since this would be theft as well as geneivas da'as. (See Even Yisroeil (9, 154) who issues this ruling concerning agents.)
In conclusion: When you purchased the computer you acted as an agent on behalf of your friend. Therefore, as soon as you purchased the computer, your friend became the owner and he borrowed dollars from you. Since your friend did not own dollars, you violated a ribbis prohibition and you can only receive in shekels the lesser of the value of the dollars you spent on the day you purchased and the day your friend paid. Finally, you are not entitled to a fee and certainly may not pad the bill to include this amount. You may ask your friend for a fee but it is entirely up to him if he chooses to pay it.
