Elon Musk’s lawyers said on Friday that Twitter’s decision to pay $7.75 million to a whistleblower gives the world’s richest person another justification for terminating his $44 billion agreement to purchase the company.

In a letter to Twitter, the lawyers said Twitter’s failure to seek Musk’s consent before making the payment to Peiter Zatko violated the April 25 merger agreement, which restricted when Twitter could make such payments.

“This severance payment … cannot be cured,” and Musk is therefore not required to complete the merger, the letter said.

A copy of the letter was filed with the Securities and Exchange Commission.

Twitter did not immediately respond to a request for comment.

The letter was filed after Musk filed under seal an amended countersuit, to incorporate Zatko’s claims of security lapses at the social media company.