Dexter SHOULD have gotten some of the money for Ed's sauce
Ed was partially responsible for the accident that made it so that Dexter had to get a summer job and pay back his teacher in the first place. I guess that one is arguable because the blame is MOSTLY on Dexter since he shouldn't have been driving in the first place, but Ed shouldn't have been in the street like that.
But more importantly, it was Dexter's idea for them to put Ed's sauce on the burgers in the first place. If it hadn't been for Dexter, Ed would have continued to bring that sauce to work every day for his own food and no one would have known about it. Ed even said himself that Dexter should get some of the money he makes.
I think the percentage was way too high - it should have been reversed, with Dexter getting 20% and Ed getting the other 80%, but Dexter was definitely entitled to a portion of that money. He should have explained that to Monique when she thought he was just a jerk taking advantage of Ed's naivete and innocent nature.
He did come off as a slimy douchebag with that contract though, and it was obvious that he was scamming Ed. It would have been better if he had approached the situation differently.