I was having a clear-out the other day, and came across my 'old' Canon EOS camera, complete with a rather nice Sigma 70-200 telephoto lens. I had forgotten about it, and had a little surge of pleasure as I realised it would fit nicely onto my current Canon 400D digital camera. Indeed it did, but when I started clicking away I repeatedly got an 'Err 99' message on the camera's screen. A bit of Googling informed me that this is a common problem - Canon have slightly altered the circuitry on these cameras, which means that some third-party lenses won't work.
I thought I would ring Sigma and see if there was any kind of a fix. A call to their Welwyn Garden City number was promptly answered by a very friendly and very knowledgeable person who told me there was nothing I could do to make the lens work, but then I got the good news. "Don't worry." he said "Send your old lens to me, and we'll sort you out with a new one that does work".
A few moments discussion and we arrived at a deal - I would get a new, £329 lens that would work perfectly with my camera, and Sigma would get my old lens and £160.
I sent the lens, got a call a day later to take my credit card details, and the new lens arrived the day after that - it works perfectly, and is optically better than the one I returned.
That's how to run a business and keep customers happy.