Here is the fan’s fantasy: You go to the ballpark and under a picture-perfect sky not only do the Yankees win, but in recognition of your exemplary behavior, the team also showers you with free season tickets, signed merchandise and a personal audience with the Yankee-est of Yankees, Derek Jeter. The team president hands you his card, with his e-mail address.

And here is the reality: The taxman may own a piece of your windfall. And not in tickets, either. He takes only cash.

For Christian Lopez, the 23-year-old fan who came up with Jeter’s 3,000th hit at Yankee Stadium on Saturday, the ramifications of his gift from above are as American as baseball, hot dogs and taxes.

As in Las Vegas, the house always wins.

“There’s different ways the I.R.S. could try to characterize a ball caught by a fan in the stands,” said Andrew D. Appleby, a tax associate at the Sutherland Asbill & Brennan law firm in New York who has written about the tax implications of souvenir baseballs. “But when the Yankees give him all those things, it’s much more clear-cut that he owes taxes on what they give him.”