Wrongfully charged for issabella Garcia account

I was paying my Issabela Garcia account through Alliance , December 2021 I made 2 monthly installation of 1200 and I paid from January2022 , march 2022 I was debited and I reversed and counselled the debit order as it was more than the amount we agreed on and the term. I later recieved calls and email that I’m owing R300 . I asked why and no one could tell what is the money for and why they wanted to charge me more than the agreed amount