Incorrect spare Key provided when car was purchased

I bought a VW Polo in 2018 from McDulings in Booysens. I traded in my vehicle at Ford last week whilst purchasing a new car only to find out the spare key provided by Mcdulings was never the correct one and never matched the car. I asked them to find the correct key but they cant. I was ******** when I bought the car and now they want R5000 for the replacement key. This should be replaced for free but they refuse. Be warned check that your spare key works when buying a car from them.