Uber employees are lining up to sell their stock to Japanese technology giant SoftBank, which will buy up to 17% of outstanding shares for $33 each. The price represents a 30% discount to Uber’s last valuation, of nearly $70 billion, but for current and former employees, the SoftBank tender offer is a rare chance to convert paper wealth into actual cash.

“Everyone’s been reconnecting to discuss the tender offer,” Lane Kasselman, a former communications lead at Uber, told Quartz. “A lot of the former employees have been waiting two to four, even five years, to see liquidity, so it’s pretty exciting.”

To qualify for the tender offer, participants must have at least 10,000 Uber shares and be “accredited investors,” an SEC designation (pdf) for wealthy individuals. Current Uber employees can’t sell more than half of their stake; there are no restrictions on former employees. The deal is on the table until Dec. 28, and could fall through if there aren’t enough shares on offer for SoftBank and a small consortium of other investors to purchase at least a 14% stake in the company.

Working at a successful startup is often viewed as a quick path to prosperity, but the reality is more complicated. Startups tend to offer equity packages, typically in the form of stock options, to compensate for below-market salaries. But as companies like Uber have stayed private longer, most employees haven’t been able to get rich from those shares. Quite the opposite, some former Uber employees have gone into debt to hang onto shares they still can’t sell.

Two former Uber employees, both of whom left the company in 2016, told Quartz that Uber gave them just 30 days after departing to exercise their options. One of those former employees paid about $100,000 to exercise more than 20,000 incentive stock options (ISOs), plus a tax bill of over $200,000. The other paid about $70,000 to exercise about 5,000 ISOs, and then about $160,000 in taxes. Both former employees took out loans from family members to make the payments, and requested anonymity to discuss their personal financial situations.

Uber, founded in 2009, is the world’s most valuable technology startup. From 2013 to 2016, the ride-hailing company’s valuation climbed from $3.5 billion to nearly $70 billion.

“It makes you start thinking, is this a real bubble, around Uber?” one of the former Uber employees told Quartz. “Everyone I know, when they were in this situation, really thought about leaving their shares on the table, because it was so much money out of pocket.”

Under current tax law, the income from exercising ISOs, a special type of option typically reserved for executives and senior employees, falls under an alternative tax calculation designed to prevent high-earners from using deductions to avoid paying tax. Non-qualified stock options, more commonly awarded to regular employees, are taxed the year they’re exercised on the gain in the stock.

Andrea Coombes, investing and retirement specialist at personal finance site NerdWallet, calls this the “dark underside of the stock-option dream.” “Potentially they can be worth a lot of money to employees,” she said. “But as an employee who’s holding stock options and hasn’t yet exercised, it’s smart to think of those as a potential benefit, not necessarily money in the bank.”

Until this year, Uber gave former employees 30 days to exercise their options, an unusually short window of time. Options that weren’t exercised by former employees before they expired were reabsorbed by the company. Uber changed its policy (paywall) in the spring so that former employees now have up to seven years to exercise their options.

Uber prohibits the sale of its shares on so-called secondary markets. Employees who have been at the company for at least four years can sell up to 10% of their holdings back to Uber at a discount, though not one as steep as what SoftBank is currently offering.

Uber declined to comment.

Travis Kalanick, Uber’s co-founder and former chief executive, famously said he’d take the company public “as late as humanly possible.” Dara Khosrowshahi, Uber’s CEO since late August, has said he’d like Uber to go public by 2019.