Should the worshipper, who is supplicating after the final tashahhud, stop his supplication if the Imam begins the taslim, or should he complete his supplication until the Imam finishes the taslim and then make his own taslim?

It is permissible for the congregant to complete the supplication until the Imam finishes his salutation. Indeed, some scholars have permitted the congregant to delay his salutation after the Imam's salutation if he wishes to recite the prescribed supplications that the Imam omitted.

It states in Tuhfat al-Muhtaj: "It is better for the congregant's salutation to be immediately after the Imam's salutation, and he should not occupy himself with the remaining prescribed remembrances and supplications, unless the Imam has omitted them."