If your client can't pay you for the work you've done, you have to write off the whole budget or just a partial amount.
In this example, the budget has been delivered but there is still some amount left to invoice. The client, however, can't pay that amount and I'll have to write it off.
To remedy this, I've created a custom service under the name "Could not invoice" which will come in handy later when I'll track the total loss of earnings. This service is valued at -50 EUR, to cancel the 50 EUR the system wants me to invoice.
Now, the budget is invoiced 100%:
Using the same method, I can partially write off the budget. If I have to invoice 1000 EUR but the client will only pay 700 EUR, I can create a custom service valued at -300 EUR which will then change the amount for invoicing to 700 EUR.