IANAL, but my understanding is that, while you can never ask anyone to give up authorship, you can ask them to transfer copyright. I.e., you are still the author of your patch, your name should be visible as a contributor (should you want to), but you transferred the rights of that patch.
I'm not a lawyer either, but from what I understand: the rights you can transfer are limited, and you can generally only give usage rights, potentially exclusive usage rights that also exclude the author from using the patch.
However, it gets complicated quickly. For example if they decide to sell the company or take on a new investor, the author could take back the grant under some circumstances which brings its own set of challenges. Can you now simply fix it yourself by writing the same code? Either you'd violate copyright or you didn't need the whole licensing thing in the first place.
Another issue is the fee: you can agree on a fee but if it's not "appropriate", that agreement is void (to protect authors from being tricked with regards to the value of their contribution).
There's a lot of uncertainty involved, which is why companies like using commercial licenses: you can usually trust that the company selling that license has figured out all the details.
