At this point in the novella, Scrooge is a completely changed man, but Cratchit doesn't know it, so Scrooge plans on playing a joke on Cratchit. When Cratchit sneaks into the office a whole "eighteen minutes and a half" late, Scrooge growls at him in his usual manner and scolds him for being so late, and then he leaps off his stool and gives Cratchit "such a dig in the waistcoat that he staggered back into the Tank again."
At this point, just when Cratchit fears that Scrooge is going to fire him, Scrooge states "therefore I am about to raise your salary." Cratchit can't believe his own ears, for he
trembled, and got a little nearer to the ruler. He had a momentary idea of knocking Scrooge down with it; holding him; and calling to the people in the court for help and a strait-waistcoat.
What a shocking surprise Scrooge's statement is to Cratchit; he believes Scrooge has gone insane, and to further shock Cratchit, Scrooge says "A merry Christmas, Bob!" and states that he will "endeavour to assist [his] struggling family" and then he instructs him to "buy another coal-scuttle" before he gets to work.
Wow! Can you imagine what Cratchit must have been thinking.