I wouldn't give up the neagtives under any circumstances. Unless, maybe, if they bought them for a fair price. What if you gave them the negatives, they then had prints made, and sold the pictures?
If you don't plan to post them on the Internet or publish them in another way, and it appears that is the case, I'd just tell them that they'll have to trust you to keep your word about that. If husband is neurotic about this, that's his problem, not yours.
Yes, next time get a release.