He should have not kept things like this from you; regardless of whether or not he feared consequences or did not find it relevant.

Don't focus on what happened, focus on where to go from here. Let him know that you want to respect his decisions, but that you also want him to give you confidence that your future together will not be tainted by this issue.

His commitment to holding his word is commendable, but it gives you stress so see if there is a way that reduces it: perhaps you can set up an automatic payment that ends at a given date, perhaps he'll just donate the remainder of the money in one go, or perhaps he'll quit altogether if no solution can be found...

Either way it's not unreasonable for you to ask him to communicate to his ex that outside of the money he will cut ties altogether (if he really does despise her, but she still has an influence that might be the better solution for him as well. If it is, he will know it too)

I hope for you that a reasonable solution can be found for all of you.