You’re not a slave. You can say no to your boss. You’re on equal footing (though your boss generally has more power/wealth so can bully/manipulate you). It’s up to your boss as to what they do when you say no. If you’re a good employee when it comes to your usual responsibilities, then they’ll keep you.

If your boss tells you to write software as part of your job and you’re not a software engineer, you can tell them to get stuffed (or sign the contract and both parties benefit!).

Edit: I just re-read the original post and saw that the OP is a software engineer. Ignore everything I said. This sounds like a terrible idea. You’ll burn all your bridges if you pursue this avenue.