Why does Oscar laugh when Paul says he will bet 300 pounds on the horse Daffodil?

Uncle Oscar laughs because he thinks Paul's entire story is a joke. He has no idea that Paul really has three hunderd pounds to bet.

"I'm betting three hundred," said the boy gravely. "But it's between you and me, Uncle Oscar ! Honour bright?"

The uncle burst into a roar of laughter.

"It's between you and me all right, you young Nat Gould," he said, laughing. "But where's your three hundred?"

"Bassett keeps it for me. We're partners."

"You are, are you ! And what is Bassett putting on Daffodil?"

"He won't go quite as high as I do, I expect. Perhaps he'll go a hundred and fifty."

"What, pennies?" laughed the uncle.

"Pounds," said the child, with a surprised look at his uncle. "Bassett keeps a bigger reserve than I do."

Between wonder and amusement Uncle Oscar was silent. He pursued the matter no further, but he determined to take his nephew with him to the Lincoln races.