Shame on Musk for writing $54.20 into the contract then.
Just because the value of something changed after you wrote the contract doesn't mean that you can break the contract unilaterally. Musk could have gotten around this with an all-stock deal (ex: I'll give the Twitter board 100,000 shares of TSLA or whatever), or other ways to write the contract without setting a particular dollar amount.
But Musk wrote $54.20 and signed it.
And the contract even says the Twitter Board can force Elon Musk to buy Twitter and finish the deal at $54.20. So they made it crystal clear to Musk (and his lawyers) that the $54.20 price points stays, no matter how Twitter's price changes over the next weeks.