Article 123 provides a critical rule for calculating the foreign tax credit: the underlying foreign income must be re-computed using Omani tax rules. Even if the foreign country uses different standards for income or deductions, for the purpose of claiming a credit in Oman, the income must be determined according to the provisions of the Omani Income Tax Law. This ensures consistency and prevents taxpayers from benefiting from more generous foreign accounting rules when calculating the portion of their Omani tax that can be offset by the foreign credit.