David Weisbach, a former Treasury Department official who helped write the regulations governing the tax-code provision that Bristol Myers is accused of violating, agreed. PwC and White & Case “are giving you 138 pages of legalese that doesn’t address the core issue in the transaction,” he said. “But you can show the I.R.S. you got this big fat opinion letter, so it must be fancy and good.”

The current status of the tax dispute is not clear. Similar disputes have spent years winding through the I.R.S.’s appeals process before leading to settlements. Companies often agree to pay a small fraction of what the I.R.S. claims was owed.

“There is a real chance that a matter like this could be settled for as little as 30 percent” of the amount in dispute, said Bryan Skarlatos, a tax lawyer at Kostelanetz & Fink.

In that case, the allegedly abusive tax shelter would have saved Bristol Myers nearly $1 billion.

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