Is a verbal contract legally enforceable?

A verbal contract or agreement, including to do more work for more money, is legally enforceable. Of course, proving or showing it can be a problem--it can turn into "he said, she said."

If you just "want out" and there's no contract obligating you to keep working, why not just bill for work to date, per the original terms, and tell your friend you can't afford to keep working--you need the time for other projects or employment. If they want to induce you to keep working, they can offer you more--though whatever you agree on, get it in writing to avoid misunderstanding.