Russian localization: exchange rate problem for foreign-currency sales

We have run into what appears to be a deficiency in Russian localization in JDE 9.1 and are wondering if anyone else has solved this. 

The situation is when our Russian entity sells foreign goods to a Russian customer, where goods will be priced in USD, and there are pre-payments from the customer involved.

Since payments between Russian entities must be in rubles, our customer will pay us the ruble equivalent of the dollar value due, using the central bank exchange rate on the payment date.  There may be several pre-payments with different dates / exchange rates. 

However, the exchange rate on the sales order / invoice will be the one effective on the shipment date.  Statutorily, the effective exchange rate on the sales order / invoice in this situation should be a weighted average of the rates applicable to all the pre- and post-payments.  (The popular local accounting software has a function that allows users to select a number of cash receipts and associate them with a sales order; it then calculates the weighted-average exchange rate and updates the rate on the sales order before generating the VAT invoice.)