[john_moscow]

From what I can tell, it's not a very big company, so they should be flexible. So I would try to turn the tables around and get a non-exclusive deal like this:

* The company agrees to pay you for adding some extra features to the project

* You keep control and exclusive rights, but give them a license to use it (but not transfer it to others)

Edit: even if they legally can claim full ownership of the project, you have the full right to resign and go work on something else after they do so. And if this is not what they want, you will have pretty good chances negotiating a non-exclusive deal instead.

One last piece of advice:

=== DO NOT AGREE TO ANYTHING WITHOUT A LAWYER ===

Many companies would consider it business as usual to agree on your terms verbally and then slip in a clause in the contract that would completely change the balance in their favor.