Thanks both of you for the explanations. So, I guess I will just negotiate with the company about selling them the source codes with GPL exceptions. Thanks again!
Any suggestion how to price it?
Both of these are excellent questions for a business lawyer.
You absolutely should have a lawyer help you with the negotiations. That is what they do. They work with contracts in exactly the same way a programmer works with code.
Business lawyers are also very experienced at coming up with valuations for products. They know what source code is selling for, how much to ask, and how much to accept.
If you don't have a lawyer you may discover that you signed a contract written entirely in their favor that ends up leaving you exposed to all kinds of problems. Topics like IP rights, indemnity, severability, and venues are all very important and you are likely to get some of them wrong unless you have help from a lawyer who understands it.
Just like a non-programmer trying to write complex source code, a non-lawyer trying to write a contract is likely to end up with serious problems.
The cost of a lawyer is pretty cheap; in my area it is about $150/hr. Compare that with the potential cost of a bad contract, which through indemnity clauses and other problems could potentially hold you liable for a fortune in a worst-case scenario.