Don't sign it.
You're giving up your only guarantee to get paid. You're supposed to be getting paid after your completion of the work you were hired for BEFORE they get paid.
They are in a bind because you're being used as a extension of their "line of credit" and using you in a way their not supposed to....maybe dragging payment from 30 days to 60...90...180 days
In my state Subs need to be paid within 30 days by the primary contractor and as a Primary Licensed Contractor CANNOT wait to pay subs before a customer pays them. Primaries are supposed to secure their own line of credit by other means to pay suppliers and subcontractors by a bank etc if they don't have the working capital.
Subs are typically required to fill out a Lien notification, a notification that reaches the customers and banks (Before work starts) that a sub has been hired to perform work. Once the work is finished the sub signs a Lien Release stating that the work is done and they have been paid.
It sounds like right now, a customer or institution wants verification that everyone from subs to suppliers have been paid so they are not held liable by someone who has been short changed, or unpaid. Once verified they will pay the Primary
You sign, and you're telling the primary contractor's customer or institution you have already been paid.
That risk is yours
If this was happening in my state, the Contractors License Board would be all up in the Primary Contractors Business for trying to obtain a release before payment, that's illegal and they could lose their License