Thanks for the explanation. Out of the box i see this working two ways:
1. Because the payment and the aging comes from the Buy Group - it still makes me lean towards making them the Bill-to on all documents. The reason being that when you receive the payment in Business Central it will be booked against the buy group customer. But if the individual companies are booked as the bill-to, then you have a discrepancy where the payment comes in against the guy group but it needs to be applied to the different individual customers. And with this case with the buy group as the bill-to, the individual companies will be the sell-to and you will still have some information on the posted invoices that you can use for manual analysis for incentives, price levers, and commission calculations; you will have to do the analysis based on the sell-to field.
2. Alternatively, you can group the individual customers as subs into the master buy group by using the customer numbers. You would have separate customer cards for the buy group and for each individual company, and you would number your customers to group them that way. For example, the buy group no. would be buygroup, and each individual company would be buygroup-001, buygroup-002, etc., something along those lines. This way when you run your aging or any other report/analysis, you would filter to include all customers that have 'buygroup' in their number. The problem I see with this solution is that the payment will be booked against the buy group customer, but then it must be applied to invoices booked against the individual companies.
You can use other fields on the customer card to do this grouping as well. For example, create a separate unique customer posting group for this buy group, and assign it to all subs/master. When you run any reporting, you can filter to run it for that specific customer posting group only. This can work with the Gen. Business Posting Group as well.
If you have access to a developer you can also quickly add a new custom master parent field to the customer card. In this field you would select the master buy group customer on all your subs and then use this field for your grouping.