How does Scrooge trick Bob at the end of the story? What does Scrooge end up doing?

Scrooge pretends to be grumpy. Bob Cratchit comes in a few minutes late and Scrooge acts like he is going to fire him. Instead Scrooge apologizes for his behavior and gives Bob a raise.

"A merry Christmas, Bob," said Scrooge, with an earnestness that could not be mistaken, as he clapped him on the back. "A merrier Christmas, Bob, my good fellow, than I have given you for many a year. I'll raise your salary, and endeavour to assist your struggling family, and we will discuss your affairs this very afternoon, over a Christmas bowl of smoking bishop, Bob. Make up the fires, and buy another coal-scuttle before you dot another i, Bob Cratchit!"