All this is correct. However for Booking.com bookings, I also want to show the Commission amount which isnt on the sales invoice but will be on the purchase invoice for that month. How do I go about adding the value from the purchase invoices and filtered only for supplier == booking.com to get my total revenue for the month?
Reason I have a custom field is because I want to know the actual revenue per month of bookings as some bookings can be made for a different month so the profit and loss account does not show the true revenue per month.