Cyrille Sautereau contacted me on this topic, and I am writing this issue on his behalf.

In the context of the preparation of the e-invoicing mandate reform in France, we are facing a frequent business case where a transparent intermediary issues a single invoice for a final Buyer, but on behalf of different sellers (and he is the payee on behalf of all sellers also). This business case is in fact, quite standard outside France also (travel agency, water company invoices, leasing invoices with insurance and maintenance, highway toll card invoices, gazoline car invoices, …)
 
We will use the sub-lines extension where we still need a place for subtype line (DETAIL, GROUP, INFORMATION) + Line Parent ID (even if cac:SubInvoiceLine could be used, the recursion could make it more complex as more than one level can be found).
 
Thus, a GROUP line per seller (/Invoice/cac:InvoiceLine/cac:Item/cac:ManufacturerParty) will provide information for the seller and a few references dedicated to the seller and totals per seller. We also need an additional reference on VAT breakdown (Seller ID) in order to provide a VAT Breakdown per Seller.
 
Regarding VAT amount on line level and the Total amount with VAT on line level (which will be necessary for e-receipt), I have found  *
/Invoice/cac:InvoiceLine/cac:TaxTotal/cbc:TaxAmount for VAT amount on line item,

but for total amount with VAT, I have only found  *
/Invoice/cac:InvoiceLine/cac:TaxTotal/cac:TaxSubtotal/cbc:TaxInclusiveAmount,

but it implies to repeat the VAT amount in /Invoice/cac:InvoiceLine/TaxTotal/TaxSubtotal/cbc:TaxAmount, which is mandatory in /Invoice/cac:InvoiceLine/cac:TaxTotal/cac:TaxSubtotal
 
Is it the right way? Or is there another way to map those 2 amounts (which will be necessary for e-receipt also).
 
Regarding VAT Breakdown per Seller / sub invoice, we need an additional BT (or the use of an existing one if possible) in /Invoice/cac:TaxTotal/cac:TaxSubtotal, in order to refer to a sub invoice ID (referenced also on line level, which will be a key for calculation, like Reason, ReasonCode, SupplytypeCode, VAT Category code, VAT Rate.
 
The only option found currently is to use the Exemption reason in text (line and VAT breakdown) to start the text with an invoice ID between # (by convention)

 
What solutions can be proposed? 
We would like to have solutions for both UBL and CII, and I have difficulties here with UBL.