please refer the below links... hope it helps you...
This Exchange Rate field shall not be editable to ensure that there will not be any GR/IR clearing imbalance which is needed thereafter to be solved via Transaction MR11.
However, this Exchange Rate will be modified during invoice payment posting maintenance via Transaction F-53. Any difference will be posted to the gain/loss account in accordance with the setting for Transaction Key KDM via Transaction OBYC.
Please co-ordinate with your FI team.