I was offered 50percent discount

I was offered 50percent discount. The discounted amount was paid in August. I requested a letter confirming that the account is paid up and was told that the discounted amount would first be credited and I should wait 7 working days for the letter. 7 working days later I called them again asking for the document and was told that it would take a while to credit the account and that I should keep checking. Now I am getting messeges and emails where I'm told I am liable to pay the amount that was discounted!