Mtn contract cancelation, hell

Okay so i canceled my contract with mtn via email, i was surprised when my sim was loaded with the usual amount and yet i had requested to have it canceled, i then called mtn they told me they did not cancel the contract as i did not reply the email, of whether or not i want to keep my sim as prepaid or not, ok fine to avoid a lot of things i accepted that and asked them to cancel it immediately, which they did, well since it was loaded with airtime, i now owed mtn, which i gladly paid the balance, so my biggest challenge they are refusing to clear my name and give me a paid up letter, reason being they cannot allocate my payment as i used the same number, which is now prepaid, mind you ive been using this number as payment reference, all along, okay so since this payment was made by internet banking, i have proof of payment, which i have sent to all mtn departments, but they dont want to clear my payment they keep replying my email, saying i owe r527, even though the proof of payment is always attached,