Elon Musk’s pledge to sell 10% of his Tesla Inc. stock highlights the complex financial web the world’s richest man has spun around his personal fortune.

Selling a stake valued at roughly $17 billion could provide Mr. Musk, who has at times said he was cash-poor, with a sizable liquidity infusion. It could also go a long way toward helping the billionaire pay a bill likely coming due from the Internal Revenue Service. Completing these moves before year-end would come with the benefit of helping Mr. Musk avoid a possible tax increase next year.