The new pay package could grant Musk 423.7 million additional shares of Tesla stock.

Tesla's stock is climbing on Monday as CEO Elon Musk disclosed the purchase of more than 2.5 million shares worth approximately $1 billion.

The stock purchase—Musk's first open-market transaction since early 2020—comes at a critical time for Tesla: the Model Y maker's market share in the US is at its lowest since 2017 even as the company...