Foregin exchange

Hello everyone

My query is regarding foreign exchange transactions. We raised invoice for professional service to our head office in Jan-14 and later issued credit note in the same month. In Jan-14 the ROE was suppose 62Rs=1US $.

Now the company wants to again re-bill the same invoice with same  specifications in Mar-14.

Now my question is

1) What rate of exchange will be considered for the above invoice..Rate of Jan-14 or MAr-14??

2) Please justify your answer with relevant accounting standards and other regulations.

Hoping for a speedy reply.

Thanks in advance

Regards

Kiran Morab