Former President Donald J. Trump’s already sizable stake in his social media company is set to jump by more than $1 billion, as he’s rewarded with additional shares in the parent of Truth Social — the result of its stock price staying high in recent weeks.

The windfall comes at a crucial time for Mr. Trump, who is on the hook for hundreds of millions of dollars of legal bills tied to the multiple cases against him. The presumptive Republican nominee for president is also ramping up his political campaign, with the boost to his net worth bolstering his image as a wealthy businessman, an important part of his pitch to voters.

Mr. Trump is already the largest shareholder of Trump Media & Technology Group with 79 million shares, a stake currently worth nearly $3 billion. He’s now due 36 million more shares under what is known as an “earnout,” additional stock that would push the value of his stake to over $4 billion.

Earnout shares, a feature of mergers, are designed to reward insiders if a company’s stock performs well for a set period after completing a deal. Trump Media merged with a public shell company last month and made its debut on the Nasdaq on March 26. The new shares would raise Mr. Trump’s stake to about 65 percent of the company.

An initial frenzy pushed the value of the company up to nearly $8 billion, but the share price has fluctuated wildly since then, falling by about half from its peak. Trump Media has been a popular target for short-sellers, who make money by betting on the decline of a company’s share price.