Musk, the world's richest man, is also the CEO at Tesla and heads two other ventures, The Boring Company and SpaceX.
Tesla shares dropped on Thursday, as investors fretted that Musk's involvement with Twitter could distract him from running the world's most valuable electric-car maker.
Twitter shares, on the other hand, extended gains as investors bet that the new funding made the completion of the deal more likely.
CNBC first reported on Thursday that Musk plans to become CEO of Twitter on an interim basis.