This story has been updated to reflect ExxonMobil's announcement that CEO Rex Tillerson will retire at year's end.

Donald Trump's transition team announced Tuesday that ExxonMobil chief executive Rex Tillerson would be Trump's nominee for secretary of state, adding another multimillionaire business leader to the president-elect's prospective Cabinet, which already counts several billionaires in its ranks and is shaping up to be the wealthiest in modern American history.

As CEO of one of the largest and most powerful public companies in the world, Tillerson received compensation valued at $24.3 million in 2015, and he ranked 29th on a list of the 200 highest paid CEOs compiled by the executive compensation research firm Equilar. The pension benefits he will receive, accumulated over more than 40 years at the company, have been valued at $69.5 million. And in a company document filed earlier this month, ExxonMobil said Tillerson has direct ownership of more than 2.6 million shares of ExxonMobil stock, which executive compensation experts say Tillerson will presumably have to divest if he is confirmed as the nation's chief diplomat.

Yet the majority of those shares -- 2 million of them, valued at nearly $185 million based on ExxonMobil's closing share price Friday -- are not yet vested. That means that the shares have been granted to Tillerson but that he doesn't yet have outright access to them. ExxonMobil has an unusually long vesting schedule and clearly states in its filings that retirement does not speed up the vesting of those shares, meaning many of them aren't currently due to be under Tillerson's control for years.

Now that the company has announced Tillerson will retire at year's end and be succeeded Jan. 1 by Darren Woods as chairman and CEO, its board is faced with a nine-figure dilemma: Should it accelerate the vesting of those shares, rewarding Tillerson for his 41 years of service just before he could take a job that has enormous influence over the geopolitics that will affect his former employer? Or should it stick to the terms in its filings, which have been cited for their good governance standards?

Under language in the company's proxy filing, some executive compensation experts say, it's possible Tillerson could have to kiss some of that sum farewell. "Based on my analysis, I think he will have to give up everything that’s unvested without special board permission," said Robert Jackson, a professor at Columbia Law School who served as an adviser on executive compensation issues to the Department of Treasury during the Obama administration. 

Others say that's highly unlikely, especially given Tillerson's long history at the company. "I'd be very surprised if he doesn't get the stock," said Alan Johnson, managing director of pay consultancy Johnson Associates. "He's worked there forever. He's done a great job. It wouldn’t be fair that he loses it because he takes this huge government job." 

But a move by the board to speed up Tillerson's vesting of the stock awards could prove to be a political flash point.

"I’ve heard of a golden goodbye, but I’ve never heard of a government goodbye," said Jackson, noting that executives who are forced to sell stock before taking government jobs can already get special tax treatment to defer the tax burden on capital gains they earn.

Alan Jeffers, a spokesman for ExxonMobil, said in an email the company had no comment on questions related to Tillerson's nomination. The president of the company's retained compensation consultant, Pearl Meyer & Partners, declined to comment on client-specific matters.

Of course, Tillerson's nomination first has to get enough votes in the Senate to be appointed, an outcome that is uncertain after several Republican senators raised concerns about his ties to Russia. Sen. Marco Rubio (R-Fla.), a member of the Senate Foreign Relations Committee, which must act on Tillerson's nomination before it heads to the Senate floor, said in a statement Tuesday that he has "serious concerns" about the nomination. He noted that he looked forward to "learning more about his record and views," but he also remarked that "the next secretary of state must be someone who views the world with moral clarity" and is "free of potential conflicts of interest."

ExxonMobil has a mandatory retirement age of 65, and Tillerson, who is 64, was already expected to retire next year before the announcement Wednesday he would step down at year's end. But the company's proxy statement says that "unvested long-term equity awards are at risk of forfeiture" if an executive leaves before the standard retirement time, although it also notes that if an executive officer leaves at an age from 55 to 64, the compensation committee of the board can decide whether the executive retains the awards.

Yet ExxonMobil's proxy also states several times that unvested stock awards are not immediately vested just because an executive retires. It goes so far as to say “equity awards are not subject to acceleration, even at retirement, except in the case of death.”

Brian Foley, an independent compensation consultant based in White Plains, N.Y., said "the proxy is fairly point blank on this."

Many companies vest executive shares over a three-to-five year period. ExxonMobil executives have to wait much longer: five years for the first half of the grant and 10 years or retirement -- whichever comes later -- for the second half. Both Foley and Equilar estimated Tuesday, before the new departure date was set, that upon Tillerson's retirement, 358,500 of the 2 million unvested shares would vest, leaving nearly 1.7 million shares, valued at more than $150 million, potentially on the table.

Such a long vesting schedule is rare, said John Roe, who leads the data and analytics arm for proxy adviser ISS, in an email, and goes along with the company's long-term employment ethos. Tillerson, for instance, has worked his entire career at ExxonMobil.

It's also smart governance, Johnson said. "You want guys who are retiring to have a long time frame," he said, "making decisions 10 to 20 years in the future" by making them hold on to the stock for longer periods. "That’s a great design, but it runs into this conflict because he got this big government job." 

If Tillerson were headed toward a more traditional retirement of golf games and grandkids, his unvested grants would simply vest over the next few years. But because he is up for the job of the nation's chief diplomat, the vesting of those shares is complicated by requirements that prohibit appointees from participating in acts in which they have a financial interest. "There's no doubt what the best practice is," said Jackson, noting that then-Goldman Sachs chief executive Henry Paulson sold stock worth hundreds of millions of dollars in the banking firm before becoming treasury secretary under George W. Bush.

Foley also guessed ExxonMobil could find a way to accelerate the vesting of the shares, given that the proxy was written when"no one was contemplating that someone would get elected president who wanted the CEO to be secretary of state," he said. "That’s an unusual event. And the tail that’s wagging the dog is a $185 million tail." 

But doing so could produce troubling optics. "If [Tillerson] were to get a sweetheart deal because they fully accelerate the shares, that could understandably raise people’s ire," Foley said. He noted that changing the terms for Tillerson would "go contrary to the whole multiyear sales pitch in the proxy that 'we have real religion about long-term performance.' For some big shareholders, I could see that sticking in their craw. The board’s going to be caught between a rock and a hard place."

That's particularly the case in light of the far-reaching diplomatic job Tillerson would be in line to do. "This is a company that has enormous interest in America's oil needs," Jackson said. "Do we want to be in a situation where a month before, [Tillerson] received a nine-figure payment from an oil company?"

One thing that is clear: If Tillerson ends up getting the job of secretary of state, his government salary will be far less than what Big Oil paid him. In 2015, his base salary at ExxonMobil was $3 million, and he received a bonus of $2.4 million, long-term stock awards that were valued at $18.3 million and benefits and perquisites valued at $540,000. The secretary of state's salary is $203,700 a year.

Read also:

Like On Leadership? Follow us on Facebook and Twitter, and subscribe to our podcast on iTunes.