Successorship to Caliphate

Imam Reza (A.S.) was welcomed at Marw by the caliph himself and by Fazl ibn Sahl and some distinguished Abbasid nobles in a place a few leagues outside the metropolis.
After some days, Ma'mun offered Imam Reza (A.S.), first the caliphate, His Holiness seriously rejected Ma'mun's proposal. Then Ma'mun proposed Imam to accept the successorship to caliphate, the Holy Imam (A.S.) turned down this proposal also but he was finally forced to accept the successorship to caliphate.
The Holy Imam (A.S.) put forward certain conditions that he would not interfere in governmental affairs or in the appointment or dismissal of government agents.