As long as the donner 出錢之前講清楚要買什麼, as you said, I don't see anything is wrong for asking payback. If the a student doesn't want to payback, he or she can refuse to accept, and the donner is justfied to terminate the support as s/he wish.

What's is wrong for someone to give large amount of money just to buy a "thank you"?

I don't have any problem with that. Why do you?

The bottom line: the donner has no obligation to give away any money. The society should be grateful that some people do, even if there are strings attached.