In a nutshell - I've lived in
Clinical Research Associate
I see this as 4 different issues: your son going against his agreement; attempts to apply discipline and consequences by one or both parents; MIL undermining your authority, possibly with your ex's approval and taking it upon herself to withhold your child support payment.
As another poster said, you can only control your son's behavior when he is with you. The signed agreement was a great idea, and since he did not fulfill the contract you can pull it out when he returns from Dad's and require that he make good his agreement. You likely have another 5 years of negotiating with him so you'll need to make it clear that he follows your rules while at your house and they're still in place when he returns from Dad's.
Ideally your ex would be willing to work with you to provide consistency and standards for the best interest of your son. Has your ex indicated whether he is interested in doing that? If he wants to support you then its his job to tell his mother to back off and let the parents make the big decisions, she can still spoil Junior but not in a way that undermines the parents. If he won't support your efforts then you probably have to let it go, unless it gets to the point where you feel the need to take him to court. Your son is old enough to understand that things may work differently at each parents' home.
Your MIL sounds like a piece of work and I don't have any suggestions there except to keep as much distance as possible. If her son won't rein her in then you may have to accept her and
I don't understand, if you didn't want your son to have the video game, why did you buy it?
I guess you were feeling pretty desperate to offer him a $50 game just so he would eat something healthy. I would hope that it wouldn't have to come down to that if it were me but, I'm sure you've exhausted all other efforts. I would be irked that you didn't get the payment when you were supposed to but, it wasn't really the dad's fault that you didn't have some extra padding in your account. If it were me, I would take the necessary steps in order to have some sort of direct deposit arrangement set up. I would not want to be put in a situation where I had to rely on my ex-mother-in-law for anything.
Do you cook?