A buyer pays $100 million for the seller in an all-stock deal, but a day later the market decides it’s only worth $50 million. What happens?

The buyer’s share price would fall by whatever per-share dollar amount corresponds to the $50 million loss in value. Note that it would not necessarily be cut in half. 

Depending on how the deal was structured, the seller would effectively only be receiving half of what it had originally negotiated.

This illustrates one of the major risks of all-stock deals: sudden changes in share price could dramatically impact valuation.