One particularly tricky nuance of contract law is that their must be someone with legal standing who wants the contract to be enforced. If Musk doesn't think he can put the funds together without damaging businesses he cares about more than owning Twitter, then he can pretty much just invoke the $B walk away if he wants to. The Twitter CEO and board were pretty clear they did not think Musk's acquisition was in the best interests of the Company, but they were legally obligated to consider an accept a fair offer of acquisition because it would be in the best interests of the share holders.

Suddenly getting $1B without having to accept Musk as the new owner is beneficial to the share holders, and crucially tying up the company in a years long lawsuit to force Musk to acquire Twitter would be expensive, and until the contract is either completed or dissolved Twitter's fund raising options are greatly limited. Suing Musk for wanting out would not be a sound financial move and could even result in Twitter becoming insolvent. Their stock price could crash, their funds to continue the lawsuit exhausted, and most likely under those circumstances Musk would win and immediately turn around and buy Twitter for a far lower price.

With that said, the only real question here is what will Musk do under the circumstances? Funding the acquisition could cause his other businesses to suffer or even fail. SpaceX is particularly vulnerable, and it wouldn't take much to end the Borring Company if Musk's finances are in a dangerous position. If he just walks away he's out $1B, assuming they'll allow it. That can't be a super attractive option. Most likely he's going to exhaust every avenue in an attempt to raise the cash without putting his other businesses at risk. But there's a clock on that, and he could end up owing Twitter a shit load if he stalls for too long.