A man who lost his wallet at Dewsbury Bus Station was arrested after staff found some cannabis inside.
They were searching for some identification in order to trace the owner when they discovered a small amount of the class B drug.
Daniel Lindley pleaded guilty to possessing a controlled drug when he appeared at Kirklees Magistrates’ Court.
District Judge Michael Fanning heard that the 20-year-old was previously cautioned 11 months ago for the same offence.
Lindley, who is self-employed, explained that he only smoked the drug occasionally.
Judge Fanning told him: “You’ve been warned about this before but it’s clearly not stopped you from doing cannabis.
“It’ll rot your body eventually so it’s worth stopping now.
“I’m imposing a fine based on your weekly take home pay so this £10 lump of cannabis has cost you £365.”
The cannabis belonging to Lindley, of South Parade in Ossett, will also be forfeited and destroyed.