To comply with government ethics rules that require him to divest his stake, and to compensate Mr. Tillerson for his shares that have not yet vested, Exxon has constructed a creative solution: If he is confirmed, the company will cancel the shares and move a pile of cash with roughly the same value into a newly created trust with Mr. Tillerson as the beneficiary. The trust will invest the cash in Treasury securities and other permitted assets and then make payments to Mr. Tillerson gradually over the next decade.

Mr. Tillerson will owe taxes at ordinary income rates as he receives the payments, the company expects, mimicking the tax treatment he would have received if he had stayed with the company.

Tax advisers, however, question whether that is the right way to handle it.

Robert J. Jackson Jr., a law professor and executive compensation expert at Columbia University and a former Obama administration Treasury Department official, said Mr. Tillerson should owe the full tax up front because the cash will effectively be his, with little realistic chance that he will lose it.

“Everything in the trust is his property today, which means he must pay tax on it,” Professor Jackson said. The notion that Mr. Tillerson should defer the tax is “a loser of a position in front of the I.R.S.,” he added. “He should pay income tax this year.”

Here is why: Incentive compensation plans like the ones at Exxon Mobil usually mean executives receive stock with restrictions on when they can sell. In Mr. Tillerson’s case, he has about two million Exxon Mobil shares — or the right to shares — that will vest over the coming decade.