Out of curiosity, how is one able to opt out of paying ZUS?
If you're employed on an umowa o dzielo basis, there is no obligation to pay ZUS. I posted the number of the article in the relevant law a few months ago but a mod binned that post, and it's certainly against the rules here to repost binned material.
Work it out for yourself - if you are running a C.S. you pay 1,300 zl a month (at the last count) no matter what your income.
So run a dzialalnosc gospodarcza and pay 799. I do.
If you don't earn, you still pay.
If they said to you "You can't have any money this month because we don't have any", would you be fine with that? No? So why should it be OK when the roles are reversed?