A good rule of thumb is, always be professional, he might ask for your talent in the future but pay you more,, never burn your bridges because customers always remember the bad stuff, In your case confront him and ask are you going to honor the deal? If not? Good luck at your new place of business and I HOPE IT GO’S WELL. Leave him feeling like an A$$. It’s not personal.. it’s just Business.
P.S. always get yourMoney up front