You should always pay back what you owe.
Serena borrowed some money from her brother Todd which she is able to pay back now but there's a new video game she wants.
Serena wants to buy the new game.
Serena pays Todd back and then starts saving up to buy the game.
Serena uses the money that she was going to pay Todd back with to buy the game.
Todd is glad that Serena paid him back and Serena feels good to be out of debt.
Todd is annoyed that Serena is being irresponsible and refuses to lend her money again.