I've read advice before that says you should ask for the parts back to, as jj says, show that they did need replacing, that way they will (apparently) be less likely to try and fix stuff that isn't broken just to get you to pay more!
In the company I work for we have exchange parts, but we have to advise the Customer of this. They can either have a brand new item at full price, or an exchange item at a significantly reduced price. If they opt for the exchange part (which they usually do) then the faulty part then becomes the property of my company and will be sent back to the manufacturers for refurbishment and will eventually be sold again as an exchange part.
So, suppose you fill the fuel tank with the wrong fuel,ie. diesel instead of petrol.The garage drains the fuel and without prior agreement they dispose of the fuel.Do they then owe you for the value of the fuel.
So, you wanted to keep the diesel fuel that you put in a petrol tank by mistake? Why?
Undoubtedly the fuel would have been a mix of diesel and petrol which was already in the tank. What were you planning to do with it? It's no use to you. Be thankful that they disposed of it for you.