If the ownership and risks is still with the manufacturer, the manufacturer can use inventory transfer to handle the situation and then capitalise all the manufacturing cost by the other party in bringing the inventory to sellable form ( Add freight-in to inventory item costs ) Just create a new location that will represent the parties location and transfer the inventory there using the inventory transfers tool.
You have the ability to create many companies on manager.io, you can create a company and activate only Goods received and Goods delivered and raise those documents from there which will not affect your inventory balances in your main company, I believe this will work fine.
I support the idea that Manager.io gives the user the option to let entered Goods receipt and Goods delivered notes affect inventory balances or not, but until that option is available you can open another company and easily import your inventory and customers and suppliers there and issue the notes from there without any problem.
The current Goods receipt and delivered note settings with inventory is still preferred by me