I imagine that if a ‘donor’ has money invested in a delegate to vote a certain way that there’s some way of checking on what they actually did. Just my thinking on the subject, though. I know I wouldn’t pay for something and have to pay and have no idea whether it ever got done or not.
I have witnessed such ‘donors’.
There’s a lot of PR pressure to ensure that not all delegates are in their 50s and 60s and relatively wealthy and at least semi-retired. Wealthy party loyalists do pay expenses for younger and less-well off delegates, alternates, and young people observers. The most common use is parents paying for a child. I have never heard of a donor demanding to control the delegate’s vote, which in most states isn’t possible anyway as the delegate doesn’t even necessarily control his (first ballot) vote.