Hello Lorenzo,
It seems to be overconfirmation situation,
You can use the cumulation rule in Transaction OVZ2
total of all receipts is checked against the total of all issues (confirmed quantity or requirements quantity) and in the case of an over-confirmation, no quantity confirmation is made.
Or you can use the backorder processing functionality to reschedule the sale order confirmation as per the supply elements situation.
Best Regards,
R.Brahmankar