We recommend working with a negative entry, as the authorities have decided that the subsidy must appear as a negative amount on the invoice.
Step 1: registering the subsidy
Create a sale with a positive free entry for the amount of the subsidy.
Select as customer the organisation that will pay the subsidy.
Settle this sale using a receipt. This ensures that the invoice numbering remains correct and continuous.
Step 2: processing on the customer's invoice
Create a free entry while keeping the description, so that the invoice clearly mentions the subsidy.
Set the amount of this free entry to €0 (so no negative amount).
Then register a payment for the subsidy amount with payment type "other".
The customer will receive an invoice for the remaining balance.