Little Adam Bhana, who is thought to be nine years old, sent the letter to Town's director Sean Jarvis because he had been taught not to keep money which isn't his.
He found the banknote near his seat while watching the side's famous win over Manchester United last weekend.
And the youngster even suggested that manager David Wagner donate the money to goalscorer Aaron Mooy as a reward for his performance.
The Australian international joined Town for a reported fee of around Â£8million in the summer, so presumably isn't short of a fiver - but the midfielder was so touched that he has offered to meet Adam in person.
The letter read: "Can you please ask Mr Wagner to give Aaron Mooy the money because he played very well and scored."
Sean Jarvis Tweeted an image of the letter with the phrase 'pure class from young Adam'
Social media users have suggested that the boy should be offered the chance to be a mascot for the club at their next home match.