A contract needs a couple of things in order to be valid:

1. A meeting of minds i.e. both parties agree on something.

2. Consideration; usually money. (This is why you sometimes see donated things sold for a dollar.)

When faced with signing a new contract that asks for more of something, I'd recommend asking for consideration of some sort. (1)

"Oh, new contract? Let's talk about my new salary."

(1) At least if you're an in-demand software developer.