You could say: To make him stay, the manager assured him of a raise. That is to say that the manager assured him that he would get a raise. You would not say that the manager assured him a raise. (The word 'of' is essential there). It would be better, more common, to say that the manager promised him a raise. You also would not say that the manager 'ensured' him a raise. You could say that the manager ensured that he would get a raise, but that means something other than what the sentence is giving. To say this would mean that the manager did something to guarantee the raise.