Lol you really think hes gonna tell HR that someone let him keep a tip?Yes it could be. A manager can't ask you to do something that goes against policy. If you didn't report him/him you could be fired to. If it doesn't feel right it's always best to ask HR.
Sure, it might be against policy but no one is going to turn down free money someone slipped them in a parking lot block by cars so no one could see