The payment clause is lawful and behaves like pay-when-paid
It never says the words, and the money still does not move until somebody upstream has been paid.
What's happening?
Pay-when-paid is largely outlawed, so nobody drafts it any more - they engineer it. A due date pegged to events upstream of your control, assessment windows that stretch, cycles offset so your money always sits one certificate behind theirs: each clause defensible on its face, and the combined effect is the old clause reborn. Cash arrives when the payer's cash does.
You price this or you negotiate it, but first you have to see it, which means reading the payment clauses as a calendar rather than as prose: map the worst-case days from your application to your money, and the engineering shows up as a number. That number either gets negotiated down before signature, or it gets priced into the tender as the cost of funding their cashflow. Whether a clause crosses the line into unenforceability is a solicitor's question; what it does to your cash is a commercial one, and it comes first.
The solution
The clause read as a calendar, then negotiated as one.