Hires in iProcurement / Purchasing - Quantity, Price and Duration

All,

Can anyone recommend a successful method of processing the procurement of a hired object?

Example: Hire of a Compressor, require 2, each costing £150 and need them both for 5 days.

Can you advise how you would enter this in an Order/Requisition?

Note: The Compressor is an non-stock Inventory Item and therefore has a fixed description, payables ivoice matching will require to be match both the quantity of the item the duration.

I would normally multiple the Quantity with the Duration to give me the Quantity, but this is causing confusion for Suppliers who aren't clear how many "Compressors" are required and for how long...