Changing the agreement

I had an accident with the rent to own car from Planet 42,the access was R12000 and I made an arrangement to pay R4000 for 3months,in November I paid R4000 and this month (December) they deducted the whole amount irrespective of our arrangement. I phoned them and they promised to refund R4851 and continue with our arrangement. Today they have changed saying that they will not refund the agreed amount, they will only refund R551,I don't know what made them to change our agreement. I need this money asap....please help.