Allowing negative quantities in invoice lines

We are using Dolibarr for our publishing house and we need to create sale invoices to our wholesaler based on their report.
The problem is that we must insert in some lines negative quantities for the books which are not sold and are returned to us.
We have already set up the global options to allow negative lines and totals in the invoice, but the “quantity” field seems locked as it allows only positive numbers.
Is there a way to solve this?

Thanks in advance!

Try creating an extra line field [Complementary attributes (lines)] depicting returns, if that is easier.


Thank you very much!