Paying for others mistakes

They sold us the WRONG BED after we very SPECIFICALLY asked for one that could take 120kg per side. We ended up with a bed that only takes 90kg per side. It started sagging badly. They agreed to exchange it but is demanding we pay in extra on other bed. It was their mistake and now we have to pay for it. No exchange will be done unless we pay in the extra. It is not fair because they made the mistake. I will not recommend them to anyone because the service is not up to standard and they make customers to pay for their mistakes. Not our fault they do not know their product