QB uses "Quantity" for determining closure of a purchase order. For similar orders, such as Blanket or non-routine Purchases, we do the following to keep the PO open:

Upon Creation of Vendor PO

Quantity Field = Total Amount for Blanket Order

Rate = 1.00

Total will still be the same.

Applying an invoice to the PO:

Received invoice for $300

Go to Receive Items and Enter Bill

Quantity Field = 300

Cost = 1.00

But you will have to Manually close the PO once completed (but doesn't matter as it it non-posting anyhow)

Hope this helps some!