You can use standard Job module in NAV. On new Job, you need to add customer 'own company'. You need to set own company as 'system customer'.
Customer must to have configure 'Payment Method Code' and also must to have posting groups:
1. Customer Posting Group (specific off-balance account)
2. VAT Bus. Posting Group (no VAT)
3. Gen. Bus. Posting Group
3.1. Sales Account (specific off-balance account)
3.2. Sales Credit Memo Account (specific off-balance account)
3.3. COGS Account (Interim) (specific off-balance account)
3.4. COGS Account (cost account)
3.5. Inventory Adjmt. Account (cost account)
All posting will be post on off-balance accounts and will have 0 balance.
Costs will be posted. This is example and you can set different posting groups.