Exchange rate curreny in custom field

I create a custom doctype name “Letter Of Credit”. Here I get Purchase Orders items child table data in LC Items child tabl on select Purchase Order No.

My System Default Language is BDT. I create a Purchase Order with USD currency. Now from Letter Of Credit doctype, when I fetch data from Purchase Order, rate in lc items child table shows in default current symbol(BDT), but PO items rate was in USD. Here are the screenshots:

Purchase Order:

LC Items from PO

Is there anything I should do for fetching the same currency here?