Kammala jumla mai zuwa: A wannan matsayin, a cikin Yuli-Agusta 803, Qasim ya jagoranci hari a cikin Byzantine Asia Minor. Ya kewaye kagara mai mahimmanci na Koron, yayin da aka tura laftaninsa al-'Abbas ibn Ja'far ibn Muhammad ibn al-Ash'ath don kewaye wani kagara, wanda Larabawa suka sani da Sinan. Duk da haka, Byzantines sun ba da damar sakin fursunoni Musulmai 320 idan ya tafi, kuma ya yarda. A watan Fabrairun 808, lokacin da Harun ya bar gidansa Raqqa don yawon shakatawa na biyu zuwa Khurasan, ya bar Qasim a baya a matsayin mataimakinsa a Raqqa, tare da Khuzayma ibn Khazim a matsayin mai ba shi shawara. Bayan mutuwar Harun da hawan al-Amin a 809, an tabbatar da Qasim a matsayinsa na gwamnan "al-'Awasim" da "jund Qinnasrin", amma a bayyane aka cire shi daga gwamnatinsa na Jazira (Upper Mesopotamia), inda aka nada Khuzayma ibn Khazim a maimakon haka. A ƙarshe, a cikin 810, Amin ya cire Qasim daga dukkan gwamnoninsa don amfanin Khuzayma, kuma ya kawo shi ya zauna a ƙarƙashin kulawa mai kyau a Baghdad. Ba da daɗewa ba bayan haka Amin ya hana ambatonsa a cikin addu'o'i, tare da Abdallah al-Ma'mun, wanda ya tashi a kusan tawaye a Khurasan. A cikin
yakin basasa da ya biyo baya, Ma'mun ya yi nasara, ya ci nasara kuma ya kashe ɗan'uwansa Amin a 813. Nan da nan bayan haka, an tsige Qasim daga matsayinsa na magaji.