If the $44 billion deal between Elon Musk and Twitter falls apart, either side may have to pay the other $1 billion, according to a securities filing on Tuesday.

The world’s richest man struck a deal on Monday to buy the social media company for $54.20 a share. Mr. Musk, who also leads the electric carmaker Tesla and the rocket maker SpaceX, has said he plans to take Twitter private and that he wants to improve the product and promote free speech on the platform.