Must the expiation for an oath (kaffarat al-yameen) be paid in cash, or is it permissible to fast, even though one is able to pay it in cash, but that would take a long time?

The expiation for an oath is one of three options, given as a choice: feeding ten poor persons, or clothing them, or freeing a slave.

If one is unable to do any of these, then he must fast for three days, consecutively or non-consecutively (though consecutively is better).

Allah the Almighty says: "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 persons from the average of that which you feed your [own] families or clothing them or freeing a slave. But he who finds not [the means to do so] - then a fast of three days. That is the expiation for your oaths when you have sworn. And guard your oaths. Thus does Allah make clear to you His verses that you may be grateful."

According to the majority of scholars, the expiation must be fulfilled immediately.