MBD looking for ways to add charges

I recently made a payment arrangement on my totalsport account. I was due to pay R1500 a month for 3 months, the last installment being due on 31st July 2019. I was also due to receive a discount of 20%.

The installment amount was calculated accoridng to my affordability, but this month i could only afford R600 due to unforeseen circumstances.

I have a letter which stipulates that if I settle later than 31st July my discount is forfeited.

My 1st installment was due on 31st May, I was paid the R600. However MBD has now forfeited my discount after clearly indicating that this would ONLY happen if I settled after the due date of 31st July. I intended to make up in the next 2 months but now MBD has gone against their word.

No where in the letter does it say I will forfeit if I don't pay the full installment every month, it says as long as its settled in the 3 months by the due date AND I HAVE THIS IN WRITING. I am now being told it's forfeited because I didn't pay the full installment, so why wasn't this also given in writing? I still have every intention of settling by 31st July, and as far as im aware the only condition on the discount was the final settlement date being met.

It is completely ******* to place conditions on a payment amount unless its in writing. I have all the letters including the conditions and i will take legal action if my discount isn't granted by the conditions i have in writing.