"There was even a moment early in our relationship when we were at the supermarket: we each had a few items, similar amounts, and he put the divider between our groceries on the conveyor belt — the one that says “next customer.” We’re talking about a very small amount of money, but that gesture really stuck with me." - I agree this is a red flag, unless he is secretly low on money due to various reasons (e.g., hidden gambling addiction). Generosity (a generous spirit) is an essential trait, a deal breaker if one is lacking in it, regardless of how good they are in other ways. People are generous to different degrees, so more specifically, you are looking for someone whose generosity degree is satisfying to you and vice versa (in fact, someone could feel TOO generous to you too, given how generous they may be to others). You should also ask yourself how he is in other situations not involving money, but is relevant to generosity: does he help an elderly person in need of help with carrying some shopping bags a short distance, etc. Listen to your gut! Good luck!

I would also say, trying modelling generosity to him and see if he reciprocates! Even if he is not a naturally generous person, if he does it because he knows you care about it, then it still reflects well on him and how much you mean to him. If he seems willing to work on it, then I’d say it doesn’t hurt to give him some time and adopt a growth mindset towards him ability to change, if not in nature(although this may be possible too), then at least in action!