Unauthorized double Installment deduction

I made a payments arrangement with Wesbank for them to debit half the installment over 3 months. They confirmed the arrangement in writing. They comp**** in November but then resumed to debating the full amount on the 22nd of December. Upon inquiry on the 24th of December, I was told that I neèd to call on the 28th to request a refund for an error that they are responsible for. A payment arrangement was made for a reason and now I have to deal with the consequences due to their incompetence yet when a non payment is made they bombard clients with communication threatening them with a negative credit bureau profile and daily interest on the outstanding balance!