if the receiver merits my post, I will have to cancel the donation, but it hasn’t happened so far
What if you receive merits after sending coins? Or what if a different post will be merited instead, just to bypass the rules? The receiver would need to put you on a merit blacklist, to be fully safe.
Merit can be cycled (especially between us), but can't be bought/sold!
Merit trading is discouraged, but cannot be easily detected in practice.