The original purchase price minus the current market value is how much they lose if he backs out of the deal, so that's the ballpark of what they would want to get paid to let him not buy Twitter.
That’s not true, they lose more. If it is acquired they sell all their shares instantly without any negative effect on the price of the shares. If they did that not as part of an acquisition, the share price would tank.
Why would the current market price have anything to do here? Twitter want him to be forced to buy for $44B, not a penny less.