Hi,
Why exchange rate difference post to G.L account in PO? Just configure KDM with ERN and ERA with required G/L account for exchange rate. More refer
http://scn.sap.com/thread/3375246
Now in your case now- for currency SGD and USD- the difference caused the exchange rate maintained in OB08 for the date with respecting to posting date of documents where matching the GR/IR clearing account and nullifying the same resulted in goods receipt and invoice receipt posted documents.
Regards,
Biju K