what do you do with the currency exchange rate difference between when the bill was entered and the bill is paid

That is fine and is accounted for in the Currency Revaluation record as a realized forex gain/loss that is automatically created when the task/page is run (assuming Foreign Currency Variance Mapping feature is enabled if I'm not mistaken). I believe there may also be cases where the realized forex gain/loss is included in the GL impact of the Bill Payment (depending on the account setup).

@CD thanks