We are a supplier (Calif) and I sent the prelim notice as soon as we received the purchase order. This is a custom item and will not be delivered for another month. The customer is telling me I have to sign an Unconditional Waiver & Release on Progress Payment for March and either a Conditional or Unconditional Waiver & Release on Progress Payment for April. I have not billed them for anything yet and will not bill them until the unit has shipped to the job site. Why do I need to sign either of these if I haven't billed them anything yet?