Gently turn down the offer.
As long as you stand by your opinion: Either you get decently paid or he go engaged in another professional.
Your gf will love you for that. Girls don't like their guys giving in to anything unreasonable.
They may throw up a fuss, but as long as you can handle it, you will be scoring in their heart.
In fact by giving in, you are not showing that you are capable of defending yourself. No girls like a guy who can't protect themselves cos then how to protect the girls?
If your gf think otherwise, that's only b'cos she is still immature.