Treasure Trove by Dick King-Smith

Ben is interested in money – not so much in spending it, but collecting it. He can’t believe his luck when he finds a bag of gold coins under the floorboards outside his bedroom. Mr Garter, the local coin dealer, says they are eighteenth century Spade Guineas and could be worth a lot of money. But unless they are declared “Treasure Trove”, Ben may have to give the coins up. Luckily, Mr Garter thinks of a way for Ben to keep them.