Pay to downgrade?

I signed up for Mweb LTE in November 2019. Due to financial contraints I cancelled the service at the end of April giving them the month of May as 1 month notice. I was then advised that I needed to pay R1500 for the balance of the modem I received. I then advised that I dont have the funds to pay this and asked if I could downgrade my account so I could still honour my agreement to pay the modem off over 24 months. I was told that the downgrade would attract a R400 charge from Telkom. Now this does not make any sense to me as I am trying to save money by downgrading, but I have to pay to do so. If I don't pay the account I will be blacklisted.