I am NOT an attorney.
#1 Your agreement was it writing or verbal?
#2 If your x paid alimony of, for example,
$1,000 per month then you would have to report alimony income of $12,000 and he would be able to claim $12,000 deduction. If he paid $900 taxes by giving you the money to pay the taxes then your income would have been $12,900 and his deduction $12,900. That is what I meant by upward spiral. What you should have done is make sure that the alimony amount received, if that was your only income, was sufficient to pay the income taxes in addition. If it was not your only income, then a proportional share of your total income.
#3 You might possibly consider a suit for the $900 per year that is not outlawed by statute of limitations in your state. The you would pay taxes on whatever you received as income.
From your posting it appears that you
basically forgave the $900 per year, not perhaps in a legal sense, but on emotional grounds.
" I never pursued it because he was being a good dad to the children ". IMO you might consider leaving it with that thought.