To me this is clearly a hint that Montresor is already fed up with Fortunato's insults and hurts, and will clearly seek his revenge later in the story. My second choice will be when Fortunato says "the cough is nothing; it won't kill me. I won't die of coughing", to which Montresor replies "True, true" (Poe). By saying this Montresor also gives us to understand that there is a plan coming to Fortunato