Refuse a refund

I had an account with Taljaard for tax purposes for R728, 00, i could not pay the account , i was handed over to debt collectors ,i sign a document which says including everything , money for collection ,sherrif money and etc..that was R2181 , then i paid R900 in cash over deposit to the bank , they then garnishe me in July with R2457 it, it was paid in full up to now month end January , then i went there to ask for my refund tge R900 that i paid as cash in the bank ,, because when the garnish me the should have take all the money that i owe them i also believe that in that garnish they even included the interest which is 9% pa.They refuse to give me my money back , they calculated so many figures deducting from my money until i was left with R350 of which they gave it to me , but to my knowledge i overpaid them by R900. please help on this matter