SOLVED. Children's fiction, at least 20 years old. Boy invents computer game and becomes rich. [s]

[Sophie]

The boy in the story is off school with an illness and in boredom writes a computer game. Someone buys it for a large amount of money, so the boy becomes rich. He agrees with his mother that we will only spend the interest gained on the money. At one point, he is shopping in a department store of some kind, and becomes overwhelmed/worried at the amount of items he's bought, but he thinks to himself that he hasn't broken his promise and has only spent the interest.

[Becca]

Matt's Million by Andrew Norriss?

[Sophie]

It must be! Thank you