When 'Abdallah died in 844, the Caliph al-Wathiq originally appointed another Tahirid, Ishaq ibn Ibrahim ibn Mus'ab, as his successor in Khurasan, but then reversed this decision and confirmed Tahir as governor.
When 'Abdallah died in 844, the Caliph al-Wathiq first selected Ishaq ibn Ibrahim ibn Mus'ab to be his successor in Khurasan, but then changed his mind and chose Tahir instead.