how to fairly divide the sale of our house with my ex?

My ex-husband and I divorced in 2005. I stayed in the house with my children and as per the divorce agreement have to sell this year. I am having trouble figuring out how to divide fairly. My ex was supposed to pay 1/2 the mortgage till we sell. In our divorce it was recorded as $125 weekly as alimony so he would get a tax break. In 2006 he told me he could no longer pay it so I took over. Through the years it amounts to me paying $50,000 for him. Also in our divorce agreement, if anyone moved in with me, my ex could charge him rent. I had someone move in in 2007 but ex did not ask for rent (he says because I was already paying his half of the mortgage). My question would be, is it legal for me to say my ex earns his money on the sale of the house on the balance of $107,000, which was what the mortgage was when he left and not on the $75,000 balance that it is now?