Legal Question in Civil Litigation in California

Parents' agreements to waive interest on arrears is not enforceable in court. Forgiveness of past child support amounts, and of interest on the overdue amounts, is unenforceable. (In re Marriage of Sabine (2nd Dist. 2007) 153 Cal.App.4th 1205, 1217.) �Consequently, notwithstanding changed circumstances, or a claimed lack of clarity in a court's order assessing child support arrearages, courts have no authority to waive or forgive interest accrued on past-due child support amounts, just as courts cannot retroactively modify or terminate the arrearages.� (In re Marriage of Hubner (2nd Dist. 2004) 124 Cal.App.4th 1082, 1089.)

Your County's Child Support Services Department, does have the authority to compromise arrears. I believe that it is for small amounts, however, and if they have refused, that means you don't qualify.

The error in your thinking comes from thinking that you and your exwife can get into a binding agreement to waive past support. But the support was for your child, that you are legally required to support, and not for your ex wife.