If the share price is lower than $54.20, it’s because the market doesn’t believe the sale will be completed. The difference between $54.20 and the current share price represents the probability of the transaction falling through.

Twitter has done nothing to impede the transaction, so any uncertainty (and thus decline in TWTR share price below $54.20) is due to the actions of the buyer, who is… Elon Musk.

Therefore, Elon is responsible for any decline in the share price since the purchase agreement was signed.