First to Fast Excerpted and adapted from a shiur by Rav Moshe Yitzchok Weg January…
Purchasing Items from the Store
A neighbor borrows money to go shopping at the local supermarket. The lender asks the borrower if they can buy an item for him as well, and adds another few dollars for the purchase.
Is the lender transgressing any prohibition of ribbis by imposing this request on the borrower?
Possibly. The Yerushalmi actually uses such a case as an example of ribbis devarim. However, the Taz explains that this prohibition only applies when done in a manner which creates “pressure” on the borrower to comply. Since this can be the case many times, the lender must be extremely careful to phrase the request in a way which will not create a pressure on the borrower to fulfill it.
An example of such pressure might be when the lender requests the borrower go to a different store “on their way” to the supermarket, or if the borrower had not planned on coming back home until much later. Or if he asks him to purchase something heavy which the borrower will have to shlep back. Another example quoted in Shulchan Aruch, is where the lender asks the borrower to stand outside and see if their friend is coming. These cases warrant extra care in order to avoid the issue of ribbis devarim.
If the borrower has done similar favors for the lender in the past, it would be permitted in any case. If he had not done so before, although he definitely would have, had he been asked, it should not be done (except in a soft–spoken manner which does not create pressure).
It goes without saying that the lender is forbidden to make the loan conditional on his request being fulfilled (i.e., “My father doesn’t really want me to lend money, but if you get this for me, I guess I can lend you the money for a day or two…”).
If the lender pays the borrower for the tirchah (effort) of purchasing the item for him, it would be permitted.