DONT SELL THE HIGH RES.
Find out specifically what he wants it for. If he wants to make a print, make it for him and have him pay for the print. Here is my print pricing...http://www.motorsportsimaging.com/prints.html
you can adjust accordingly to what you think, but I wouldnt go below about $30 usd for an 8x10 size.
If he wants the file for a screen saver or something like that, then you can include it free if he buys a print and size it to his screen size and save it at about a 5 jpeg. That will keep him from making lots of extra prints.
If he doesnt want a print at all, find out what size he wants, size it and save it as a 5, put your name on it and charge him like $10 or something and maybe he'll come back to you next time when he wants prints or other work.
But just dont sell or give away the original because you will lose all control over the image and they can do whatever they want with it including making unlimited prints or whatever. You need to keep control, you wouldnt have just sold the negative in film days right?
Keep control, figure out what they want and price accordingly.