I think eventually, Manager will have to be able to allow payments to be made from different accounts on the same Purchase Invoice, or allow users to resource multiple invoices to create a report. ( Like a " Combine Multiple Invoices " option )
For example, one can have a single shipment coming in from the Supplier, with different Inventory Items in that shipment. And, this shipment will be handled by a Freight Agent who is completely separate from the Supplier, yet, we all want to include the cost of freight on the items and have it shared across all the items to allow us to easily calculate the profit margin after sales, while still being able to have separate Purchase Invoices for each Supplier within the main Purchase Invoice, sort of like mini Purchase Invoices, for each supplier.
I suggest the following solutions.
Why not have a sort of " Shipping Draft" section that will allow users to merge all the resources that went into delivering a shipment. This " Shipping Draft " section can be linked directly to the Inventory Profit Margin Report section allow users to generate reports. Meanwhile, the Purchase Invoice section stays intact.
On the Inventory Profit Margin Report section, you can allow users to combine information form a set of Purchase Invoices to generate a report. This way, users can add a Suppliers’ Invoice + a Shipping/Freight Invoice = Cost of Goods, and so on.
Next to the Inventory Items on the Purchase Invoice while in " Edit " mode, could be a menu that allows the user to select the Supplier of the individual item, this will allow Manager to be able to generate multiple invoices from one purchase invoice.
One of the foreseeable downfalls of all of these options is that they will rely heavily on the accuracy of the Exchange Rate module thats built into Manager in the event that for example the Supplier and the Freight Service operate on different currencies.