Is a father considered foresworn in his oath and does expiation become binding upon him if he swore not to lend his son money, then the son borrowed an amount from his brother, and the brother borrowed the same amount from the father to return it to the son? And is a friend obliged to fulfill his friend's oath which compels him to an act that causes him bodily harm?

If the father and son conspire for the son to lend money to his brother on behalf of the father, then the father is considered to have broken his oath, because this is tantamount to him lending the money, and stratagems are impermissible. However, if the father lends money to the son without conspiracy, and then the son assigns the debt to his brother, the father does not break his oath, because he did not directly lend to the other son, and an assignment is not a loan. You are not obligated to do anything that harms you just because your friend asks.