If they want to pay you, payment is within 30 days after the invoice is sent, not "when we have cash". That's a joke, not a contract.
This is more true than I can tell you. Even when it comes from "friends". Once you agree to these 'terms', you will go right to the very bottom of the list. There will always be something more pressing to spend money on than to pay you for something that is already done. They'll also find a way to make you feel bad for asking even if you do it while they're unpacking their new Aeron chairs and 24" monitors.
I have traveled this road, friend. Here be sorrows.