Mr. Musk is Tesla’s largest shareholder and owned roughly 17% of the electric-vehicle maker, or more than 172 million shares, before this week’s sales, according to FactSet.

He is on the hook to come up with $21 billion in cash to finance the Twitter deal. That funding plan also includes borrowing $12.5 billion from loans backed by more than $62.5 billion worth of Tesla shares that he owns. Tesla and several banks have put in place rules that would require him to put up more collateral if the company’s share price falls.

Mr. Musk tweeted Thursday: “No further TSLA sales planned after today.”

The Tesla boss didn’t immediately respond to a request for comment about whether he sold shares in the company on Thursday.

Mr. Musk has a net worth of $252 billion, according to the Bloomberg Billionaires Index, making him the world’s richest person. But much of his wealth is tied up in his companies, which include Tesla and Space Exploration Technologies Corp., or SpaceX. Mr. Musk, who is compensated in stock awards and doesn’t accept a cash salary from Tesla, at times has described himself as cash poor.

As of last year, more than half of Mr. Musk’s Tesla stock—or roughly 88 million shares—was pledged as collateral to secure personal debt, an August regulatory filing shows.

Mr. Musk was for many years reluctant to part with Tesla stock. However, he sold more than $16 billion worth last year, much of it to pay taxes due on a large tranche of vested stock options he exercised before they would have expired this year.

Payment Patchwork

Elon Musk is paying for Twitter partly with his own cash, and the rest with a mix of a buyout loan and a loan against his Tesla stock.

Where Mr. Musk is getting $46.5 billion to buy Twitter

$12.5 billion margin loan against Tesla

$12.5 billion margin loan against Tesla

$12.5 billion margin loan against Tesla

$12.5 billion margin

loan against Tesla

$12.5 billion margin

loan against Tesla

Selling stock could weaken Mr. Musk’s control over Tesla. Companies such as

Facebook

parent

Meta Platforms Inc.

and Google parent

Alphabet Inc.

have multiple classes of shares, giving founders supervoting power over common shareholders. Tesla lacks such a structure.

In addition to the shares he owns, Mr. Musk has tens of millions of vested Tesla stock options. He is in line to receive billions of dollars worth of additional stock options after the electric-vehicle maker reported record quarterly earnings earlier this month.

Those payouts are part of a massive 2018 pay package that rewards Mr. Musk when Tesla hits certain performance metrics.

Shares in Tesla, the world’s largest car maker by value, are down roughly 19% since the last trading day before Mr. Musk disclosed his Twitter stake.

Write to Rebecca Elliott at rebecca.elliott@wsj.com