Praise be to Allah.
Kafaarat yameen (expiation for breaking an oath) means freeing a believing slave, or feeding or clothing ten poor people. Whoever cannot do that must fast for three days. The basis for that is the verse in which Allaah says (interpretation of the meaning):
“Allaah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn”
If a person swears many oaths before breaking the oath, and the thing sworn about was the same, then he only has to offer expiation once. But if he swore many oaths about different things, then he has to offer expiation for each oath.
Kafaarat al-yameen must be given to the poor, as stated by Allaah. There is no reason why it cannot be given to the mujaahideen who are fighting for the sake of Allaah, because Allaah has mentioned them among those who may be given zakaah, especially if there are no poor or needy people to whom it may be given.
It is not permissible to pay cash instead of offering kafaarat al-yameen, because that is going against the text. Your Lord is not forgetful and He did not state that cash may be given instead of the expiations mentioned in the text.