purchase order

Hi,My client has started a new business  of construction  and they want to map it in JD. In this the work order that is (manually prepared), and given to sub contractor. Here in the Work Order the order Quantity is No of Sites. but the payment is done, once the  supervisor goes to sight and check that how much meter of excavation is done.
Depending on the length of excavation in meters the payment is done. This payment also is not fixed. It might be anything from 10% to 90%.
So work order is made in no of sites and payment is done on the basis of length.