Is it Islamically permissible for charitable organizations to discriminate in salaries between Saudi and expatriate employees, by granting Saudis higher salaries despite equal experience and qualifications, or even the expatriates sometimes having superior ones?

The basic principle is that there is no obligation to equalize wages among employees, as contracts of hire are based on mutual consent.

However, since the questioner is the manager of a charitable institution, it is not permissible for him to pay wages exceeding those of the existing and appropriate workforce, because showing favoritism to some workers contradicts the best interest of the institution.

An exception to this is if the institution is governmentally mandated to employ citizens at specific salaries; in such a case, it should adhere to the minimum required.