I have a case study, and i really need to know the answer for…

I have a case study, and i really need to know the answer for following question

Walt, a recent immigrant to Australia who still does not speak English well, owns and operates a small printing firm.

Some time ago, Walt took a 5 year lease of a printing machine from Wonderworks Pty Ltd at a rental of $5,000 a month. The lease will expire in 2014. Due to the economic downturn there was increasingly less work for Walt’s printing business. As a result, Walt was having difficulty meeting his debts as they fell due.

Meanwhile, Wonderworks Pty Ltd was having financial difficulty of its own. Five of its clients were declared bankrupt and others were having difficulty in meeting their lease payments.
Eight months ago, when the manager of Wonderworks Pty Ltd learned of Walt’s difficulties, he sent him a letter stating: “Wonderworks Pty Ltd will be happy to accept $3,000 per month as full payment of the lease until things get better in the industry”.

Walt has paid $3,000 per month for the past eight months. During that period he decided to expand his business to the framing of pictures, as well as the printing business. He bought equipment for the framing work and is committed to paying it off at $1000 per month for the next three years.

Recently, business in the printing industry started to improve. Wonderworks Pty Ltd sent a letter to Walt requesting that he start paying the full $5,000 per month.
Walt wants to know hislegal position. He asks your advice on the following two questions:
1. Can Wonderworks Pty Ltd demand the difference between the contract price ($5,000 per month) and the amount Walt has actually paid ($3,000 per month) over the past eight months (ie: $16,000)?
AND
2. Does Walt have to start paying the full $5,000 per month in the future?