I am a GC on a project. The contract notes the GC is to submit the final and unconditional lien waiver in order to establish substantial completion.
May 21, 2020
Ultimately, final unconditional mechanics lien waivers should only be exchanged after final payment is made. After all, that's the point of the waiver - it acts as a receipt showing you've been paid and no longer have lien rights. Though, if there's retainage or other outstanding amounts, then including those amounts in the Exceptions section would make sense.
So, regardless of what the contract says, it's not a good idea to waive lien rights prior to payment. And, generally, payment won't be made until final completion of the project (though that could certainly change from project to project).
Discussing the matter with your customer should help to straighten things out, especially if you've already got a steady relationship. Offering up a final conditional lien waiver prior to full final payment might be a fair middle ground.
As a final note, this resource should be useful: Should You Sign That Lien Waiver?