Let's say that Mario Pret's room will be added to Nathalie Rey's reservation.
Click then on Nathalie's reservation
Click on the number of the reservation
Copy this number. It is the ID of the invoice of Nathalie's reservation
Click on Calendar
And click on Mario's reservation
Click here
Click on Add to Invoice
Click on Search
Type in "40" which corresponds to the invoice of Nathalie
And click on Search
Select Nathalie's reservation
Click here to add Mario's room to Nathalie's reservation
And click OK.This is how you can transfer articles from an invoice to another