Overpayment of court order

by Ginger
(Katy, Texas)

I am a custodial parent receiving child support from my son's dad. He was ordered to pay a given amount of 469.00 a month and 20% of any bonuses received from his employer. This came to a total of about 650.00 a month. He paid this faithfully for several years. Three years ago, my son's father calls me and tells me he's changing jobs and that his new salary will be different. He tells me he will be paying 740.08 to me now. I received that payment for 3 and a half years. My son is now in college and still receiving payments from his dad in the amount of 469.00 monthly. I had to call his dad and remind him that he was to continue payments through August 2014 as per our decree. He went back to the original amount in the order of 469.00 and sends that. He has also consulted an attorney who has sent me a letter demanding payment of right at 10K where he's over paid me for the last 3 1/2 years. Am I responsible for this overpayment? I was over paid per decree, but not per his salary. In the 10 years I received child support from him, there was never a modification do through the courts, only from the dad himself. Do I need to hire an attorney for this, or will it just go through the state.