Terry contracts with Dave to paint Dave's house for $4,500. When the job is less than half finished, Terry gets a great job offer in Adelaide and tells Dave "I'm sorry I can't finish your job as I have accepted another job offer in Adelaide". Dave is very worried about being able to move into his house on time and says to Terry "I'll pay you an extra $1,000 if you can finish painting the house before you go to Adelaide." Terry agrees to this proposal and finishes Dave's house as promised. Dave pays Terry only $4,500 and refuses to pay the other $1,000.

What is the law of consideration in Terry's case?