Is it against a prohibited oath?
Someone may think that this narration mentions the prohibited form of oath. It is not so. Muhaqqiqe-Qummi says that in the first place the chain of narrators of this tradition is weak. Secondly it may be that ordering such an oath may be the sole prerogative of an Imam. The Imam (a.s.) knew that the one taking such an oath is not a believer. He was an enemy of Ahl ul-Bayt (a.s.) and it was necessary to resort to such means in order to clear himself of the false allegations.
(Jame ush-shatÄt)
From the explanation of this tradition by Muhaqqiq it is clear that Imam (a.s.) considered that accursed man deserving of death and his death only depended upon the oath of dissociation. It was also necessary that Imam (a.s.) was not disrespected and that the Imam (a.s.) could save himself from being unjustly persecuted at the hands of Mansur. Another benefit that accrued was that, Mansur for the time being, refrained from oppressing the other SadÄt (descendants of Holy Prophet) and believers.