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.
After 'Abdallah's death in 844, the Caliph al-Wathiq first appointed Ishaq ibn Ibrahim ibn Mus'ab as his successor in Khurasan, but then changed his mind and chose Tahir for the position instead.