I have done the reverse.

I will do freelance work on any project for 1 day for free, as long as:

* It is a start to finish project. I won't finish off someone else's work or take on something I don't think I can get into a working state in a day. These are usually MVP type things.

* It's available to the public. No company internal tooling.

* The requestor will be about on the day. I want them to see me work.

This policy has gotten me lots of later paid work, which I price pretty high.

I consider it more as a demo of my abilities, and the first day of any project is usually the one you learn most in.