When does the exchange rate take effect?

Issue/Requirement:

A job has been ongoing for several months during which the exchange rates have changed. When does the exchange rate take effect

Solution: 

The job depicts the selling currency and rate (these can be overridden on sales invoicing).  The job has a currency so that quoting can be done in currency.

The purchase estimate/order depicts the buying currency and rate (this can be overridden on Purchase estimate and Purchase order).

Purchase invoices are also entered in currency with a rate for the PI.  If the rate is different between the purchase invoice and purchase order, then you will effectively be over or under allocating the sterling amounts.

So they are all separate.