Fixed rates in Project Billing

Hi,

question about fixed rates in Project Billing:

I have a customer which has his funding in foreign currency

and agreement exchange rate is the corporate rate on 25-Aug-2009.

I want to have the same  rate in all invoices based on this agreement

and the rate on 25-Aug-2009 in spite of the invoice date or invoice

creation date. I have certain setups done for this,

but not working just now,

thanks for your help,

seija loikkanen