Foreign Currency procurement cycle

Hi Experts

Suppose we purchase import Material

1) Purchase order rises 47 USD

2)when invoice came that time Currency rate increase

3)at the time of invoice which rate i have taken

4)the payment which value i have to pay

5)in this procurement where we change the rate and how foreign currency valuation impact in accounts in sap systems

Plz entire procurement scenario that i can explain to my client how to foreign currency valuation

happen in sap system