Small changes after invoicing

A solution could be found to allow us to make some changes, for example to modify the number of people in the reservation that are already invoiced, because we receive the money before the arrival and we are obliged by law to invoice. Many times the client reserves for a number of people and then arrive less and if we do not change it in the reservation gives us error when sending the data to mystery of interior, and if we try to change it tells us that you can not make any change with invoice made. Or that the number of people does not appear on the invoice, I do not know if you have any idea to solve it, greetings.