Adam is on the correct route, but not quite right. It is a bit more complicated than that. It depends on which account you set as your COGS account, not the Sales account.
You have to do the correct setup in the “General Posting Setup” table Be aware of which accounts are debited and credited in the GL. A Sales account would be credited with the selling price, while you need the expense to be debited. I have a similar situation. We sometimes give stock to customers as samples / gifts / or use it in PR events. The customer does not pay for it at all, but I want the cost of the stock to reflect as a PR expense. I have setup the accounts so that the COGS account field, in the setupt table contains my GL account for PR expenses.
When I do the sale, I put a R0.00 selling price, thus when the Invoice is processed, there is no revenue, but the cost of the stock posts through to the Expense account.
Remember for each Debit there must be a credit. In this instance Inventory is Credited and Expense Debited at the value of the stock eg. 100.00. Thus if you have to take a snapshot of your Income statement at this point, you will see a loss of 100.00
If you had put a Sales value of 100.00 in, whatever you put as a Sales account will be credited, and “Receivables account” (As on Customer Posting Group Table) will be debited, thus you will show that your customer is owing you money at that point, but your Income Statement will show 0.00
You are not clear as to the processes that follows, or at which point you actually do “Bill” your customer
The whole point that I want to make, is that it is completely possible to predetermine into which GL account you want to post costs
You can even make it more interesting, in that it depends on the customer / stock combination, to which GL account it posts.
I have managed it very successfully.
Very importantly though, you need to ensure that whomever sets it up knows what they are doing.
Play with a couple of scenarios, and trace it through to your GL to see if you get the desired effect.
There might also be other solutions eg. bill of materials, where you can create an Sales item, link all the components to it via the Inventory BOM, and then post it.
Or even the Manufacturing module, where you “use” Items to make other items, which you sell.