child support

I have paid child support faithfully as required and one time using one of your lawyers I increased the amount as I had earned more. I have earned significantly less and according to the guidelines I e-mailed the other parent that I would be lowering the amount for each month to which she e-mailed me back agreeing to this. after the first payment she said I cannot change the amount but I have an agreement between us with our correspondence by e-mail. Am I able to use the e-mail correspondence as proof of both parent’s agreements on this? Any help would be appreciated thank you