renting with opp.to buy 4 yrs later how can agency get away with it?

4 yrs ago i gave agency $2500.00 down on this house believing after 4 yrs if i could'nt get a bank loan , they would help me get one since part of the rent was going to a fund to help buy this place.Now they know and knew i could'nt get a loan by myself they asked if i still want to live here renting since i cant get loan. I am getting ssi disability and raising kids alone 1 also getting ssi for bipolar. what can i do? legally i mean.