Thanks to Abd al-Malik's influence with Harun, Qasim was named as third in line of succession in 802 or 803, shortly after the so-called "Meccan documents" which established the precedence in succession of his elder brothers Muhammad (the caliph al-Amin, r. 809–813) and Abdallah (the caliph al-Ma'mun, r. 813–833).
In addition, Harun entrusted Qasim with the command over the frontier provinces with the Byzantine Empire (the al-Thughur wa al-'Awasim), with his seat at Manbij.
[4] After Harun's death and the accession of al-Amin in 809, Qasim was confirmed in his position as governor of the al-'Awasim and of Jund Qinnasrin, but was apparently removed from his governorship of the Jazira (Upper Mesopotamia), where Khuzayma ibn Khazim was appointed instead.
[5] Eventually, in 810, Qasim was removed by al-Amin from all his governorships in favour of Khuzayma, and brought to live under close supervision in Baghdad.
[6] Amin shortly after forbade his brother's mention in the prayers, along with Abdallah al-Ma'mun, who had risen in near-revolt in Khurasan.