Base set I bought matress is sagging.

I bought a base set at Joshua Doore last year April and in 5 months the mattress was sagging i went in and report then they changed now again the same mattress is sagging and when i report the supplier Bravo Group sleep product they telling me that its out of warranty but i am still paying the Bed even now.Today i got a statement from Bravo sleep products stating I must pay R426... then they say We therefore cannot authorise the exchange of the faulty product until the amount due has been paid. Bad service and I will never ever paid that money while i still pay the instalment. They must give another Bed or i am taking this matter to credit ombudsman.