how to make payment of multiple forien currency invoices with user exchange rate on invoice through

@USD

Hi

we have a case where there are multiple foreign currency invoices to be paid @ user defined exchange rate on invoice.

for example--invoice 1--of 01-jan-2013 @USD 50 Rs/dollar

invoice 2- of 01-mar-2013 @USD 45 Rs/dollar

now we want to pay both these invoices on 01-jun-2013 at the exchange rate at the time of invoice creation. means, on 01-jun, invoice 1 to be paid @ 50Rs/dollar and invoice 2 @ 45 Rs/dollar.

how can we achieve this through payment batch ?

Please advice

Best regards

soham