Mammon knew that stealing from another witch would bite him in the ass...He had cleared all his debts with the other ones, but he just couldn't help himself and tried to steal from you...You caught him, of course...

Your punishment for him was just binding him to you in case you ever needed protection...That was surprisingly tame, so he thanked you profusely while you smiled...That's the moment he believes he fell in love with you...Now, all he needs to do is get you to feel the same way...