"I think I mentioned once in an early story that he earned that coin by shining some man's particularly muddy shoes. Wish I could remember the issue number, etc. Was his client a miner? A nobleman? A banker? Can't recall.
What you say about the coin being an American dime does bring up the possibility his client was a visiting tourist. And that could cause young Scrooge to be suspicious of the strange coin's value. This would be a good time to start building the dime's mysterious power.
Scrooge's every attempt to trade the coin for negotiable ha'pennies, or whatever, could lead him into a luck-blessed situation that enriches him to some degree. By the time the dime has helped him amass enough moola to buy a ticket to America, young Scrooge has become aware of the coin's beneficial influence on his fortunes. But does he buy a ticket to America? Hell no! He works for his passage across the seas -- perhaps by shining all the sailors' shoes with his old shoe shining outfit.
I don't know how the above fantasizing jibes with your planning of the plot business. I only included it to demonstrate the fecundity of the plot you've invented. Gimmicks and gags seem to grow in it like compound interest."
On April 22, 1991, Barks continued with a story-idea that went into the whereabouts of Scrooge's Old Number One being a lucky dime:
"I agree with you that the Number One Dime should not be treated as a good luck charm. It contradicts the way Uncle Scrooge really made his fortune, but woe is me! I blatantly violated that rule in at least one story, U.S. #46, "Lost Beneath the Sea." I not only had Scrooge calling the old dime a "boodle-bringer," I demonstrated such powers at points in the story.
It doesn't preclude your writing a story that debunks the luck charm misconception. One way might be that the dime gets stolen by the Beagle Boys, who figure that it is a good luck charm that will attract all sorts of undeserved wealth to their wallets. Needless to say, no matter how well they plan their bank heists, all the dime will attract is swarms of cops. Meanwhile Uncle Scrooge is having a terrible time. His stack of money shrinks a few inches every day. In desperation he even buys lottery tickets that never seem to win. It looks very much as if the old dime was the gizmo that made him the richest duck in the world. His luck is gone kaput.
Then he changes suddenly. He says, "Luck! I didn't make my fortune by being lucky. I made it in the old-fashioned way! By hard work." So he goes back to the hills with a pick and shovel and lots of sweat on his brow and before long he has a flock of new gold mines and oil wells and is richer than ever.
Walking along the street one day he is wondering how the Beagle Boys are making out with his old dime. He soon learns. The B-Boys, passing in a paddy wagon on the way to jail, bean him with the dime.
Now Scrooge places the dime in the position it is fitted for -- a memento of the way he got his start. He relaxes in his money contented. His overloaded money bin needs no more money. He personally needs no more money. He has got it made.
The doorbell rings. It is Don and the kids bringing news that one of the ten-cent lottery tickets that Uncle Scrooge thought worthless months ago has been declared a belated winner. Scrooge will have to make room for ten million dollars -- all in dimes."
Don Rosa didn't use this story-idea. In chapter 12 of the "Life and Times of Scrooge McDuck" the crooks do make off briefly with Old Number One, but the coin shows no sign of either blessing or cursing them, while Scrooge's response to the whole luck myth is vociferous denial: "'lucky' dime?! What thimble-headed gherkin invented that supreme bit of absolute balderdash?!" This debunks "the luck charm misconception" very directly, instead of Barks' indirect way by leaving the conclusion up to the reader's perception.