How to change Currency

Dear Sir,

   When we receive the Invoice with Eur Currency,             But at PO cur is at US .  How can we change pay the  vendor at Eur Currency. 

for Eg : $100 = Eur 80  on that PO date and when the price variance occurred due to global market then  $110 = Eur 80.  on the Invoice date.  How can we solve this problem.

   regards,

   Surendranath.