Be prepared for him to bombard you with calls if he feels he is being rejected by you or if you stop his allowance. He is so used to everything being on his terms and treating you so badly that it has become normal for both of you.
Be prepared for him to make you weak at the knees, turn on the charm, and if that doesn't work get angry and aggressive with you.
Start to keep a journal and write everything down just in case he gets nasty as you know he can be. Or you can write it down here, whatever you do, keep it open, do not allow him to weaken you. You know he will try.
Stand up for yourself. Do not get bullied by him anymore. Tell him you have been researching Islam and what he is doing is dishonorable (if he upsets you and you don't know what else to say.)
Whatever you do, keep yourself safe.
Ask him for the money back and ask him for money (turn the tables and see how he reacts). Say you need the money to go on a course or something/help out your parents as they have been conned.
I think reverse psychology would work quite well here op, be brave.
Write out/record a few scenarios/ in front of the mirror to prepare yourself.
Good luck! You have all these mumsnetters on your side op, you have to take control of your situation. You can do it!!