By Drew FitzGerald

Sprint's former chief executive Marcelo Claure will own another $500 million of T-Mobile US Inc. shares as part of a complex deal that will help SoftBank Group Corp. wind down much of its stake in the enlarged U.S. cellphone carrier.

The deal, weeks in the making and earlier reported by The Wall Street Journal, will allow SoftBank to sell about two thirds of its stake in T-Mobile through a process that could raise about $20 billion, based on recent share prices. T-Mobile shares slipped 1.5% after hours Monday to $105. The stock sale is expected to price later this week.

Mr. Claure, SoftBank's chief operating officer and a current T-Mobile director, will also buy 5 million shares of T-Mobile, T-Mobile said in a securities filing Monday. The personal acquisition will be funded by a loan SoftBank is providing Mr. Claure, who struck the original merger agreement with T-Mobile and ushered it through several antitrust challenges. His total stake will rise to about 6 million shares after the deal closes.

Mr. Claure was a billionaire entrepreneur who had sold control of his cellphone distribution business to SoftBank when he joined Sprint's board in early 2014. He took over as Sprint's CEO in August 2014 after the company had abandoned a prior effort to merge with T-Mobile.

The company mostly struggled to gain customers and post profits for several years. Mr. Claure struck a new deal in 2018 to merge with T-Mobile. Soon after, he stepped down as day-to-day Sprint CEO and joined SoftBank as an executive. He remained Sprint's chairman until the deal closed and then joined the T-Mobile board.

The Wall Street Journal last month reported that SoftBank was seeking ways to pare its yearslong investment in the U.S. wireless sector to meet its goal of raising $41 billion. The process would also allow T-Mobile parent Deutsche Telekom AG, which already controls the company, to take a greater share of its subsidiary's common stock.

T-Mobile closed the deal to take over Sprint in April. The arrangement gave SoftBank a nearly 25% stake in the company, subject to certain lockup provisions. The Japanese company soon entered talks with Deutsche Telekom to alter the agreement as it looked for ways to raise cash.

The deal announced Monday also provided T-Mobile a $300 million fee for allowing the stock sale, which won't increase the company's overall capitalization. T-Mobile said director Ronald Fisher, another director appointed by SoftBank, will leave the board.

T-Mobile's former chief, John Legere, left the company in April. He received severance and stock awards valued at more than $100 million after spending years reviving the company and negotiating the deal with Sprint.

Write to Drew FitzGerald at andrew.fitzgerald@wsj.com