I might just add somewhere in there that "I have spoken to an attorney and been informed that:"....
That way it is clear that you are not just making assumptions but that this is the law.
Maybe like this: It has come to my attention that this month’s payment from you for D and myself is only xxxx instead of the agreed xxxx.
I have spoken to an attorney and been informed that the terms of the Separation Agreement cannot be arbitrarily changed by just one party, but have to be mutually agreed and ratified by the court.
I started to pay half of the mortgage at the start of the year even though we did not go through the court. But this unilateral action on your part is not acceptable.
I invite you to pay the remainder into my account by the end of this week (by June 24th), or I will be forced to advise my lawyer.
I would also like to inform you that the payment of xxxx for the month of August, when D is with you, is also to be paid as usual, and cannot be held back.
