They’re saying their letter was a mistake and are asking for more money

I was in arrears with the CSA FOR OVER £3000,when it came to the ex wife giving me my shareof the home we agreed that i would take £1,500 less and she agreed that she will not pursue the rest of the debt, i asked to have this in writting from the CSA confirming this,which they did but they still came after me for this debt

I have just finished paying this after 18 months, and now they are saying i still owe over £1,500 i have sent them the letter they sent me,but they areignoring this saying the letter was a mistake,wha can i do?