Personally I would simply refuse and prepare for the possibility that this company will no longer be a client... or charge more for the inconvenience and change in relationship status. I presume you freelance because of freedom to choose clients, projects, and terms.

If this is really just your employer (only client), but you have a "freelance" relationship for tax purposes or whatever, then you might want to consider whether you will be better off just getting a job.

As a software engineer, I assume you have employment options (there is demand everywhere it seems), so you can probably afford to do what makes you happy.