How to: Tiered Conversion rate for Invoices and Payments?

We need to calculate the Exchange rate during the Payment creation process.

We have a tiered exchange rate. 

For example, 

   $0-$1,000 has a rate of currentExchangeRate*1.2; 

   $1,001-$5,000 the rate is currentExchangeRate*1.1 and 

   $5,001-$10,000 the rate is currentExchangeRate *0.9

We created an Invoice on Jan-01 for $2,000 with an exchange rate of x. We need the rate to be x*1.1

When we make the Payment using the Payment Manager on Jan-15, the rate changed and is now (x+.5). We need the rate to be (x+.5) * 1.1 . 

We would like to get the Exchange rate again at that time. How do we get the exchange rate during runtime? Our currency exchange provider has an API that we can call but I cannot find documentation on how to insert the new rate in the Payment Manager screen.