Another thing to really worry about is to make sure that you actually have the rights to all of your code and that someone didn't include code that isn't yours in your distributed sources.
You should definitely at a minimum take an MD5 of the code that you make available to them so that you can later prove that file 'gpldcode.c' wasn't a part of what you sent.
A listing of exactly what it is that you license should be part of the deal, filenames, linecounts and descriptions, there should be a 'non compete' clause, you should think long and hard about the conditions under which you want the license to be invalidated (those may cause the deal to not go through).
Licensing source code is a lot harder than licensing a binary.
http://www.google.com/search?q=licensing+source+code+agreeme...
Will give you a whole pile of reading material, I'd still advise you to retain a lawyer that specializes in IP matters for your jurisdiction. Lawyers are expensive, domain experts more so, if the amount is small then the risks may not outweigh the upsides, if the amount is large enough it makes good sense to do the deal and retain a lawyer.
Is there any chance at all to modify the code to their specifications and license it as a library module that they will link to?
That might save you a large amount of headache.