What should I do about promises I am no longer able to fulfill, and some of which I have forgotten?

If you vowed to give a certain amount of money to someone upon the fulfillment of a condition, then you must fulfill the vow once the condition is met. If you are unable to fulfill it, an expiation for breaking an oath becomes obligatory upon you. Similarly, if you swore an oath to that effect, it is preferable to fulfill the oath, and if you are unable to, an expiation for breaking an oath becomes obligatory upon you.