Agreement on payment of account and mistake on amount paid.

I have had an agreement with Debt Tech that i would pay them R200 per month on an account of Afhco (African Housing Company) i have been doing that and end of last month i by mistake paid the wrong amount to the account, this amount was my rent amount and what was due to them i paid into the Rent account. Afhco notified me of this and i sent them the mistake i had done. they said they would sort it out. a week later they said that they were not able to help me anymore and i had to contact Debt Tech myself. mind you they are a debt collector for them. i did so. debt collect is not now willing to reimburse the amount paid and get what has been agreed on with with so that rent is paid and only agree that they pay the balance from what i owed. afhco deals with them and this i think should have a issue that could have been resolved as per agreement done on account.

i am standing here not knowing what to do as i am not getting help from none of these companies that do dealing together. first of all i am paying this account (debt Tech) because of their own internal affairs not in system, i dealt with a person who is said to be no longer with them now. so since that person is no longer, it is said that they do not and could not deal with it. so i agreed i would pay and did so.

i need help in this please.