Child Support?

My ex husband is suppose to pay 300.00 every other week for child support. Originally we had custody set up on every 2 weeks alternated. Now he told me we could reduce this to every other week *if* I would return the chid support to him. I did this because my kids were so miserable with him and his new wife. Now he is wanting me to write a letter that is notarized showing I pay this back and always will until the kids are 18. He wants this because he is trying to finance a house. If I do this to keep the peace, will this hold up in court, should we ever go back to court?