There's two ways to avoid double-charging yourself on the Hire Desk rehires, and I have gone into detail below.
Method 1: Order "On Behalf of":
When raising the rehire, enter the hyperlink on the line number on the sales order when assigning the assets to the requisition:
On the "Rehire" tab of the following webpage, check the box labelled "Order on Behalf of".
Order on Behalf of:
What this option does, is when the items are dispatched, it creates the Procurement Plant PO but does not create an internal dispatch. In terms of financial implications this is the same as not raising a hire on the hire desk, because it creates a purely external commitment. Subsequently, this stops the hire from being charged on the Hire Desk and Procurement/Purchase Ledger.
This hire is then be managed entirely from Procurement | Plant Orders but will still be visible on the operational reporting from the Hire Desk module.
Method 2: "Unlinking" Rates:
Another way we can achieve a similar result (where there is not charges posting from Plant Control, and is all managed by Procurement | Plant Orders and Purchase Ledger) but still retain the hire management functionality from Hire Desk is to unlink the rates when processing the Rehire. This stops the Plant Order element of the process from being locked to the rates entered by the user for the item on the hire desk.
When processing the requisition and dispatch, enter the hyperlink on the Hire line (same as Method 1).
Instead of enabling "Order on Behalf", disable the "Linked" checkbox, and enter the suppliers' price:
Once that is done and the supplier details and cost have been entered, press "Save", and then "Next" to go back to the Requisition lines view. We can see here that the Charge Price is blank, but the supplier price is £568.00p/WK
This setup will mean that any charges for this rehire against the contract from Plant Control will be £0.00, but the Procurement Commitment will be £568.00 a week when the line is dispatched and PO committed.
The second method will let the Hire Desk manage the on/off hires for the Rehire PO, whereas the first method will remove most of the Hire management from the Hire Desk operatives and instead put the onus on the people managing the Procurement Plant orders.
To resolve the current issue with the cost hitting the GL twice you will have to journal the costs back; there isn't a supplier account that a manual credit can be processed against. The alternative is to cancel all transactions and work back to the unconfirmed dispatch, where we could enact of the above methods.