Wrongly charged

I paid 48000 the 30th May to speed up paymemt on my outstanding vehicle instalment. I was left with an oustanding balance of about 2300. I assume FNB wesbank will debit on our agreed day as usual. In June I was saw a tax rebate of about R400, and left with a contract balance of about R1895and realise that they did not deduct.. I then paid the R1895 on the 16th of Jul since I had some fund but when checking my app Wesbank added R1400 my contract outstanding balance changed to about R3400 without explanation.. a letter was sent on 11th July showing a contract oustanding balance of R7000.. just being **********, im not paying the amounts that you also dont know where they come from, my account is soo up to date .. and a large payment reduced interest. Im so dissapointed.. you like reaping people off.