Is expiation required for someone who swore to give money to his father, then borrowed the amount and gave it to his father, whereupon the father refused the amount and gave it to the son's brother, who in turn absolved the son of the debt?

If the oath by Allah was to pay a sum to your father, then you have fulfilled your oath simply by making the payment, and his refusal to accept it does not harm you unless you intended for him to accept it. In that case, you would break your oath and expiation would be due, unless he accepts it.

If the oath was by divorce for the same purpose, then if you did not intend for your father to accept it, you have fulfilled your oath simply by making the payment. However, if you did intend for him to accept it, then you have broken your oath and the divorce has occurred, unless your father accepts the sum. For intention specifies the general and restricts the absolute in oaths.