What is the expiation for someone who swore to pay the price of something for a friend, but the friend insisted on paying and paid himself? Is it obligatory to give the money to the friend even if he refuses it? And is the expiation due through feeding or fasting, or is the oath considered a vain oath for which there is no expiation?

Whoever swears by Allah to do something, and makes a binding oath upon it, then does not do it, must offer an expiation for the oath (kaffarat yamin). The expiation for an oath is: freeing a slave, or feeding ten poor people, or clothing them. If one cannot find any of these, then he must fast for three days, based on the words of Allah the Almighty: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten poor people from the average of that which you feed your [own] families or clothing them or freeing a slave. But whoever finds not [the means to do so] - then a fast of three days. That is the expiation for your oaths when you have sworn."