Who’s signing whose contract?
If they are signing yours, then what it says should be honored. Payment within set time parameter.
All of their other vendors expect to get paid. You are no different. In fact, if there are a bunch of materials to be ordered, my vendors insist on a partial payment up front. No reason you can’t do that too.
If you are signing their contract and your payment is contingent on client payment, decide how much you need the work and if the chance of delayed payment is going to affect you. I’m gonna point out that it isn’t against the law to amend, or even strike out, a clause in an offered contract before the other party signs it. Of course doing so may mean you don’t get the job, but at least the client knows where you stand.