That distinction was crucial. Until this year, the “like-kind” provision allowed owners of similar types of property, such as machinery or fleet vehicles, to swap their assets without paying taxes on either party’s gains until the asset was sold. In a baseball or basketball trade, the assets aren’t players, they are the players’ contracts — and the I.R.S. was allowing them to be exchanged without fear of taxation.

The new law breaks that peace, by limiting like-kind exchanges to “real property,” which is shorthand for land or other real estate.

“I don’t think that they thought about baseball when they thought about this change,” said Kari Smoker, an accounting professor at SUNY Brockport who has consulted for the N.B.A. and National Hockey League players’ associations in legislative disputes over sports taxation. “It raises all kinds of issues, which I think were easier to ignore, probably, when we had a simple rule that it was a like-kind exchange.”

Mr. Verlander, for example, was clearly a more immediately valuable asset to the Astros than the three prospects they traded to get him. He gave up only four runs in his five regular-season starts for the team, then won four straight starts to begin the playoffs. In very simple terms, he brought value to the Astros in a trade, and had the new law been in place last year, the team would have owed taxes on that added value.

But what, exactly, was that value? Was it the size of his contract? Mr. Verlander earned $28 million last year, while the players traded for him drew minor league salaries. Was it the additional wins he brought to the team? Statisticians estimate Mr. Verlander gave the Astros nearly two more wins last season, a value that, depending on the statistician, could reach $20 million. Or was it some calculation of the total future value Mr. Verlander will bring to the team, minus the total future value it gave up in the prospects it traded away — and possibly adjusted for the amount the team will have to pay Mr. Verlander?

Complicating matters further is that teams value players differently, and one player might help a certain team far more than another team. A struggling club with a surplus of starting pitchers might trade one to a playoff contender in desperate need of one, in exchange for position players who could improve a struggling lineup. In that case, both teams could, reasonably, be considered to have gained value in the trade, and thus would owe taxes on it.

That is exactly how Adam Guttridge views the trade of Mr. Verlander — as a win-win for both the Astros and Tigers, which would have resulted in a capital-gains tax bill for each of them. Mr. Guttridge is a former manager of baseball research and development for the Milwaukee Brewers and a co-founder of NEIFI, a consultancy that has developed software to attach a dollar value to every professional ballplayer and his expected future performance.