Ok, I have some more info. sorry for any confusion.
The shot is very nice, imho, but not necessarily a "once in a lifetime" shot. The group interested is a non-profit group who work with disadvantaged children and homeless people. I've decided to let them use the photo for free (charitable donation and all that... I wouldn't get a huge amount of money anyway). However, I of course would like to retain all rights to the image to sell and exhibit in the future. So, is there any kind of standard contract I could use / could someone give me help with the wording of the agreement? Thanks!