Bad Service

I fell behind with my cartrack payments due to the pandemic lockdown in 2020. So I when things got back to normal I went back to work after retrenchment. I started paying off my outstanding balance because I couldn't pay it at once. Eventualy in December 2021 could afford to pay up my outstanding balance of which cartrack offered me a discount on. We made arrangements and I settled with the amount we agreed on and payed in full.

And been paying my monthly installment. two months later March 2022. Now cartrack is calling and saying that I owe them. An outstanding balance. The outstanding balance is now the discount I got from them. How I this possible?