They wont let you cancel but they will let you upgrade

I am trying to cancel an contract, which is due for cancellation yet they are informing me that I cannot cancel due to an outstanding amount, which has been paid and they claim that it has not yet been reflected on their end, I have offered to provide proof of payment, now if the contract lasts any longer, it goes out of promotion and I am charged way more, instead of assisting me with the cancellation they are offering other contracts and upgrades, so you cannot cancel until the amount reflects but you can upgrade