There seem to be a couple of opinions like that:
I think you prolly eyed his stack wrong. Cus why would a guy who obviously has money ( "he often donates 500-1000") be trying to squirrel away 20 bucks.
and I could not disagree more. Here's why:
At my home casino, there's a guy who routinely looses 5-figure numbers playing roulett and general consensu is that he can afford it, as he runs some wholesale trade business.
Once in a while he comes to play some poker, mostly 2-5 NLH, buying in for 500-1000€. Needless to say, he's a big fat tuna and regularly burns multiple buy-ins a night. On the other hand, he knows that his casino losses earn him some privileges.
If you are heads-up against him and the hand ends without a showdown, he will oftentimes try to grab your wholecards to have a look at them, knowing very well that (if he is called) the floor will do nothing about it but say: "This is not allowed...blabla."
While this is something I don't mind given his losses and prevent by tossing my cards directly into the muck, I saw him once taking a 100€ ship off his stack before making a bad all-in call. The player involed saw it as well (but no other player/dealer saw it), called him out, got the floor over, had a short chat and made him add the 100€ chip again.
Long story to back a small but important fact to keep in mind:
Just because someone has a lot of money, that does not mean he does not care about it. Also, some people just enjoy being sneaky.
So pay attention and make sure you don't get cheated or unintentioally payed short, when playing live poker.
In your chase I would probably just say something if I really saw him taking the money and even then do it in a friendly way like: "So you wanna get a massage before you try to win your money back?" After all, you want him to stay.
If the amount is like 100 bucks and more, but I did not really see him taking the money off the table, but know his stack was way bigger in the beginning, I just leave the table, go talk to the floor I know best in private and ask him to check the tapes. This avoids ugly situations.