Good morning, how could I do or what should I inherit to set the rate field (foreign currency $) to a specific value.

In my country we have 2 values ​​for the dollar currency, for the purchase 1 value and for the sale another value.

defined 2 ratios for the dollar currency:

usd-sale = 3

usd-purchase = 4

When creating the supplier invoice, I need you to take the usd-purchase ratio = 4, and use it for conversion to the company's local currency.

odoo:11

I will appreciate your support

Thanks a lot,