Not Honoring Guarantee

I purchased a sink mixer from Builders Express Bedfordview on 25/05/2020.I took back today because it a seal inside of it broke. It was inspected , and confirmed to be faulty. Then they said that the guarantee was up. I pointed out , that on the front of the box , it is clearly printed 12 YEARS GUARANTEE. I was offered a refund , and said no thank you , I want a replacement , and I had to go and fetch it myself. My receipt was then taken to the cashier , with the new mixer and was informed , that the exact product has increased in price , and I must pay in the difference. I asked for the manager , and he also insisted I pay . asked him to show me the T&C's .His response was , He need not have it in writing. I asked for his business card , instead wrote on a peace of paper ,Nk0sinathi Mncube ***