How To Contra With Different Foreign Currencies

How does A21 handling dual currency contra of accounts while booking in proper exchange gain and loss under the following scenario:  
(a)   We bill a Customer in US$ transaction. For e.g. US$1,000/- @1.700
(b)   This Customer is also a Supplier, who issue invoices us in EURO. For e.g. EURO 900/- @1.900
(c)   Customer pay in US$ to contra invoice.
(d)   How to handle this in A21?  

SOLUTION:  
1. We will need to perform this transaction as separate ones to capture the US$ to S$ as one, and EURO to S$ as another.  
2. Raising the Sales Invoice for US$1,000/-