I do this differently than Rustler; for one thing, this Invested Cost being held until production means it is never posted to Expense. That makes it WIP = Work In Process, Other Current Asset on the Balance sheet and never hitting the P&L. It hits the P&L later as part of COGS for what is sold. And what got sold needs to have been Fully Costed.

You can use the Service item in the Build, but that Service item links to the WIP = Other Current Asset account. That also requires that you listed this on the Purchase for that outside labor. And that also requires you have a Known Cost per unit. If all of that applies, then you can put that cost on the Edit Item (not a two sided item). The build will then use that Value, and the account the item is linked to, WIP, will be reduced when you Build.

Another, more flexible, method is to still list the Service Item on the purchase, linked to WIP. Now the Build does not use it. You can Build. Then, you will use the Inventory Adjustment function. You will Increase the Value for what you just put on hand from that build, by "investing" the WIP value that you know went into this Production Run. Top left, the WIP account is where you pull from, for the amount you know is in WIP for this production run.

Example:

Make one Other Current Asset account and use it for WIP as Outside Labor, Broker Fees, Import duties and Pallet/Shipping costs for a production run. Use a different Other Current Asset for Raw Materials. It's the same concept, but now you have some clarity for where you put something you incur, temporarily, until you invested it in something produced and on hand.