Wes bank Changing the agreement

I bought a car via Wes bank on the 12 February 2020. Since covid we working short hours. I’ve been paying every month now received a call from their lawyer saying my account is in arrears and she explained that since sometimes I don’t pay on the 25th at creates arrears on my account didn’t know about that. So we made arrangements that I pay R6000 for 9 months then I will be up to day bear in mind my instalment is R4470. I paid the R6000 on the 25th of November. Then I paid on the 25 of December. January received Sms’s that I should pay my arrears. I revert back to their lawyer who said she will check for me. After checking she tells me I must pay R600 more and I’m like I don’t have and the agreement is R6000 and I didn’t break that arrangement. There are not fair it’s either I continue with the R6000 or they increase my paying months.