I don't make these kinds of blanket, open-ended offers unless I really mean them (which means I don't make them). DH makes these offers and means them without thinking in advance (and, sometimes, just means them in the moment). Just yesterday, he stated he told BIL to let him know if BIL and SIL need help moving this month, and DH will help them. I asked DH how he expected to do that without taking help away from me (us) for a situation that I gave him a deadline he requested (and is costing us lots of money on a daily basis). DH realized he messed up and said that he would not be helping them then. It's on DH to figure out how to retract his offer; he will not be taking any more time away from helping us to help others while we are in this situation.
Another time, I was busy prepping food for DH's family's annual camping trip. DH was sitting around not doing anything and asked if he could help with anything. I said no then, a few minutes later, I *did* need help and asked DH for help. DH was put out as he decided he was going to spend the time doing something else (a sort-of leisure task that could be done anytime). I "enforced" his offer (to borrow a term from someone upthread). I pointed out I was getting things ready for HIS family camping trip and that he had offered to help me. DH stopped what he was doing and helped me instead. DH has a really bad habit of not offering help until I am already done with something -- DH isn't sly or malicious so I think it's just a matter of him being really clueless but, yes, it drives me nuts and, finally, I mentioned it to him a few months ago (it's not like I'm expecting help because, if I need help, I'll ask; it's a matter of feeling his offer is disingenuous because he keeps doing it when I am done, as if he's asking to get "credit" for offering when there's little likelihood I'd actually be taking him up on it). I get a sense he still gets put out when I take him up on his offers of help, sometimes, because I'll be prepping stuff to cook and he'll ask me if I need help. Most times, I don't need help so I decline. However, peeling veggies is laborious and hard on my hands so I will sometimes take him up on this. I've noticed he's not very happy sometimes when I accept, and, frankly, I'll call him out on it (because, again, he shouldn't be offering if he's going to be all annoyed that I accept his offer).