At the time of making the Supplier Invoice entry in USD, there is no conversion needed even if the company currency is in SAR except for the GL entries that would get converted in the Purchases account - check the GL entries for the transaction after Purchase Invoice entry.
Again, when making payment in USD from a cheque drawn on a USD denominated bank account, no conversion is needed except for the GL entries on the purchases / stock inventory accounts - check them as well.
You should have an Exchange Variances Account assigned in Setup => System and General Setup that will be used for Balance Sheet, P&L Account and costing / settlement purposes.
Alternatively, if you entered the exchange rate manually, try to logout and login again and then make the Supplier Invoices and the Payments to Suppliers. This is assuming you are plagued with some browser cache - set the browser options to clear cache on exit.