What is the ruling on a father's vow to slaughter a sheep upon purchasing a car, if he did not fulfill it, knowing the difficulty of fulfilling it now and his inability to recall the exact wording of the vow?

Your father must fulfill the vow whenever he is able, and it does not lapse due to the passage of time. If he is unable to fulfill the vow due to an inability that is not expected to cease, then he must offer an expiation for an oath: feeding ten poor persons, or clothing them, or freeing a slave. If he cannot find these, then fasting for three days.