Cost value not coming as per date of SO Creation exchange rate

Hi Guru's,

Need help on below issue:

While creating Invoice(VF01), we provide the DO no. in the document column

and then press enter then it gives the overview of billing items. Here the

'cost value' is getting calculated as per current exchange rate but as per

my client it should be as old exchange rate(if exchange rate has changed

during this period)

If anybody has any idea about this. Also if anybody know about any user

exit or BADI to modify the value of Cost here as per SO creation date

exchange rate.

Thanks and Regards

Abinash Kumar