nah bro, he gave her the amulet to further cement the scam. miyu didn't believe that her big sister died so he gotta give the amulet to let her know for a fact and to also scam her that he can revive the sister for a big sum of money
That scam came later. At first he gave it just to honor her wish. When she've shown them her magic capability he decided to use that and scam her then. Remember, the guy himself was abused (although it was played for laugh at that moment) until he started abusing others to survive. Pretty realistic and unfortunate reality of a lot of criminals, but does not excuse his action