UNHAPPY CLIENT!!!!!!!

MBD offered me a discount on my Cell c account and i paid the account in full and they have sent me a paid up letter i forwarded the letter to Cell C and at Cell C they told me that according to their records i have not paid the amount that was handed over to MBD i informed them that i was offered a discount but they want proof so i called MBD and they told me that they cannot help me because my account is closed on their side so they cannot provide me with anything now i'm stuck with a paid up letter that doesn't work, cell c is rejecting it due to the lack of proof that i was offered a discount