I received a letter from a customer complaining that one of my engineers told them something that turned out to the incorrect, when I speak to the engineer he says he didn't say it and I must say I believe him.

This customer is trying to make up for their own mistake by blaming us, how do I reply to him without upsetting him further. He is clearly in the wrong, but short of calling him a liar (which I won't) I don't know what to say to him.

Any advice guys?

I think I would start with:

"I apologize that you have had difficulties with this issue. The idea that one of my engineers would have given out faulty information of this sort concerns me greatly. To this end, this is what I have done; I have spoken with Mr. Enigineer, and between him and I we can not figure out how this got mixed up. Although we can't go back and find the mix up, I would like to go forward help you correct your situation. What can I do to help?"

Wear the Right Hat!
Bill