Tesla’s Frankfurt-listed shares fell about 9% in early trading on Monday as investors prepared for the chief executive Elon Musk’s proposed sale of about a tenth of his holdings in the electric carmaker after his Twitter poll.

Musk, the world’s richest person, tweeted on Saturday that he would offload 10% of his stock if users of the social media network approved the proposal.

The poll attracted more than 3.5m votes and 57.9% of people voted “yes”.

Musk had said previously he would have to exercise a large number of stock options in the next three months, which would create a big tax bill. Selling some of his stock could free up funds to pay the taxes.

“I was prepared to accept either outcome,” Musk said, after the voting ended. Market participants expected speculators would try to front-run his selling.

As of 30 June, Musk’s shareholding in Tesla came to about 170.5m shares and selling 10% would amount to close to $21bn based on Friday’s close, according to Reuters calculations.

Including stock options, Musk owns a 23% stake in Tesla, the world’s most valuable car company. In the three months to 4 November, company insiders at Tesla sold $259.62m-worth of shares, excluding dispositions of indirectly held shares, according to Refinitiv Eikon data.

Musk’s straw poll follows a proposal by US Senate Democrats to tax billionaires’ stocks and other tradeable assets to help finance president Joe Biden’s social spending agenda and fill a loophole that has allowed them to defer capital gains taxes indefinitely.

“The last thing you do when offloading a massive exposure is to reveal your hand,” said Chris Weston, the head of research at broker Pepperstone in Melbourne. “The buyers tend to step away when you have an overhang like this, but this is no ordinary story and is Musk’s way of getting back at the proposal to tax the elite with gains on unrealized profits.”

Tesla breached a trillion dollars in market capitalisation late in October, becoming the fifth US company to join a club that includes Apple, Microsoft, Amazon and Alphabet.

“I’d be looking for clients to flip to shorting fairly aggressively, although hold periods would be very short-term in nature,” Weston added.