Is it sufficient to fast three days as an expiation for an oath, if money becomes available later, after the fasting, knowing that the money will be available at a later time before performing the expiation?

Expiation is immediately obligatory. Whoever is obligated to expiate and does not possess what would feed or clothe ten poor people is not required to borrow, based on the Almighty's saying: "But whoever finds none, then a fast of three days." So if, at that time, it is true that he does not possess what he needs for expiation, he transitions to fasting. If he becomes wealthy after completing the expiation through fasting, he is not required to repeat the expiation. And if he becomes wealthy during the fast, he may continue with it according to the majority of scholars.