His simile: a king who lets a servant choose a reward decrees to give whatever he chooses
In the same place he illustrates the point with a comparison from human life. 'If a king,' he says, 'gives his servant the option of choosing and asking for this reward or gift, or that one, or a third, he certainly has not decreed to restrict him to one of them. He has decreed to give him the particular reward or gift that the servant chooses or asks for.
This can be said of God much more. The king gave the servant the option perhaps without knowing, and uncertain, what he would choose. But to God, from whom nothing can be hidden, it was certain from eternity what David would choose.'
Latin
XII. Ibidem vero rem illam illustrat simili quod occurrit inter homines. Si rex, inquit, servo suo optionem concedat hoc vel illud, vel tertium praemium aut munus eligendi ac petendi, certe non decrevit eum restringere ad unum horum; sed decrevit dare ei illud certum praemium, aut munus quod ille elegerit aut petierit. Multo magis id de Deo dici potest. Rex enim servo optionem dedit, ignarus forte ac incertus quid ille electurus esset. At Deo, quem nihil latere potest, ab aeterno certum erat quid David electurus esset.