That was then, this is now. The new mantra for the financial markets is being repeated scores of times in City boardrooms as company bosses and their financial backers have second thoughts about deals signed or committed to only a few short weeks ago. Prices that looked cheap then now look expensive. Terms that seemed reasonable then now look reckless. Financings that looked like easy money for banks then now have the smell of albatrosses that could linger unloved on the balance sheet.

Leveraged buyers of assets and their underwriters are revisiting every deal not yet set in stone and investigating whether they have any room for manoeuvre. Sometimes there is just no wriggle room, nor should there be. The whole point about paying fees for underwriting is to ensure the cash is there for the deal even if the sky caves in.

Some banks are already resorting to drastic measures to persuade clients to release them from such promises. Our exclusive story on Saturday about the banks behind the $45 billion (£22.2 billion) buy-out of the utilities group TXU vividly illustrates the point. The banks reckoned that in current conditions they had no chance of selling on the $37 billion of debt they are due to take on to their books to finance the deal. They offered to pay the $1 billion break fee, which would have allowed the private equity bidders Kohlberg Kravis Roberts and TPG to walk away at no cost to themselves. KKR and TPG said no but it is a measure of their anxiety that Goldman Sachs, Morgan Stanley, Citigroup and others were prepared to write such a big cheque for their freedom.

Asset purchasers are minutely perusing contracts to see whether they can invoke material adverse change clauses – or MACs – to walk away from deals or use them as a bargaining tool to extract better terms. It worked for the purchasers of the supplies division of Home Depot last month. The private equity buyers Carlyle, Bain Capital and Clayton Dubilier & Rice got a $1.8 billion discount on the original $10.3 billion price tag and forced Home Depot to lend them $1 billion to help to finance the deal. Their banks invoked a MAC clause to delever the financing and force the buyers to put up more equity.

HSBC, announcing its $6.3 billion planned purchase of a majority stake in Korea Exchange Bank (KEB) yesterday, was careful to spell out the MAC in its offer announcement. If due diligence uncovers anything toxic and material at KEB, HSBC wants an easy exit. But MACs are unlikely to be lifesavers for many reluctant asset buyers. They are usually only meant to be invoked when a nasty surprise specific to the acquired company is uncovered, not when the overall sector or indeed the entire market is buffetted by a storm.

Nor should they be invoked except in exceptional circumstances. The Takeover Panel rightly forced WPP, the advertising agency, to go through with its bid for the media-buying agency Tempus. It wanted to back out because of the impact of the 9/11 atrocities. The savings bank TSB did the honourable thing and carried on with its £777 million offer for Hill Samuel after the 1987 crash even though by then the deal looked a pig in a poke.

Company-specific shocks are legitimate reasons to pull out but are very rare. Godfrey Davis, the car-hire firm, was able to withdraw its offer for Sketchley in 1990 but only because fraud had been discovered in the drycleaning group.

We are a long way from the old City days of My Word Is My Bond. But asset buyers and their financiers should be aware of the reputational dangers of invoking MACs without very good reason.

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