NoDictionaries   Text notes for  
... accusatio decernitur. decernaturque et maneat provincialibus potentiam suam tali...

maneat <ius> provincialibus potentiam suam tali modo ostentandi:"let the right remain to the provincials of displaying their power in such a way/manner"; the gen. of the gerund,"ostendandi," requires, I think, a noun as head word.