You've won a contract to deliver 300 pencil cases over 3 months (100 per month). Total contract value is $300. When the contract is signed, a sales order is created for the full contract value. At the end of each month when the delivery is complete, a sales invoice is issued to the customer and upon payment the revenue is posted to accounting. You can do the same on the purchasing side by issuing the purchase invoices against the sales order. This process will also allow reporting of GST based on the accumulated revenue and costs for that quarter. For easy tracking, ensure that the contract, sales order and invoices are referenced to each other.