The tax break also exposes one of the greatest vulnerabilities of the United States tax system: it depends on voluntary compliance. The I.R.S. staff is so outnumbered by tax lawyers and accounting departments at major corporations that there is often little to prevent taxpayers from taking a freewheeling approach to interpreting and administering the rules.

What’s more, the tax break is one of so many that it tends to escape attention. The independent Simpson-Bowles deficit commission appointed by Mr. Obama in 2010 raised the possibility of eliminating it and other tax expenditures, however, and some budget experts argue that the program should be severely limited or repealed.

Some financial planners and economists say that the tax break even favors real estate investors unfairly by allowing them to defer capital gains taxes that those who invest in securities and other ventures have to pay. And although it was originally intended to help farmers, some economists and lawmakers in agricultural areas say it has perversely contributed to suburban sprawl and the spiraling cost of farmland. Because it allows farmers to avoid capital gains taxes on land swaps, the tax break provides an incentive to sell farmland coveted by developers and buy property in less desirable and more remote areas.

A crucial element of the tax break is that companies cannot use the proceeds of an asset sale for any purpose other than buying a replacement. Because the break is intended to encourage reinvestment in a business, companies that sell an asset are required to deposit the proceeds from the sale into an escrow account, which must be controlled by a third party. If the company has control of the money, the tax break is disallowed, according to tax experts.

“Funds cannot be held under the taxpayer’s sole control,” said David Shechtman, a lawyer who headed the American Bar Association tax committee that oversees the exchanges.

Documents and testimony at the recent federal trial indicate that a subsidiary of one of the nation’s largest banks, JPMorgan Chase, stood by for years as the companies — most of which were also major depositors in the bank — bent the rules governing these exchanges of property. JPMorgan sold the subsidiary in 2008.

At least a half-dozen companies were allowed unrestricted access, according to evidence at the trial. For at least 30 months, Wells Fargo had unfettered access to billions of dollars intended to be held in escrow, the records show. And American subsidiaries of Volkswagen and BMW had so much control over the money that they used it as collateral to obtain lines of credit.