A fixed asset can be acquired and fully depreciated to a net book value of $0.00. If the deduct cash discount parameter is active and the invoice is paid after the fixed asset is fully depreciated, a negative net book value can result.
The scenario is
1. Acquire the fixed asset
2. Fully depreciate it => net book value is $0.00
3. Pay for the fixed asset and deduct a cash discount
I would expect receiving a warning pop-up window that informs users about the fact that the fixed asset has a net book value of $0.00. Then the user should have the option to decide whether you wants to have the discount posted on the fixed asset (resulting in a negative net book value) or whether he wants to have the cash discount as an earning on the account linked to the discount code.
Many thanks and best regards,