If you get the bill in $, you create the bill in $.

Then, if you pay it in SAR, you register the payment in SAR.

If there is a difference between the exchange rate at the time of the bill and at the time of the payment, the difference will have to be posted in the "Exchange Difference" account.

You need to enable multi-currency to do this.

Complete documentation: https://www.odoo.com/documentation/user/10.0/accounting/others/multicurrencies/how_it_works.html