The Royal Bank of Scotland-led consortium could invoke a “material adverse change [MAC]” clause to lower the price of its €70 billion offer (£47.2 billion) for ABN Amro if market conditions worsen.

Sources said that RBS and its partners have not yet had any discussions about using the so-called MAC clause, but it is an option if the credit crunch continues to wreak havoc.

A MAC clause allows an acquirer to try to reduce an agreed price for a target or walk away altogether if it can prove that there has been change that has had a material impact on the value of the target.

They are not often invoked and are very difficult to prove, especially in the UK, where the Takeover Panel has famously ruled against them in the past, most notably forcing WPP to press ahead with its acquisition of Tempus in 2001. However, given the extent of the turmoil in the credit markets, some companies and virtually all the top investment banks are talking to their lawyers about invoking the MAC as a way of wriggling out of overpriced deals.

The phenomenon is particularly prevalent in the private equity arena, where the banks have underwritten billions of dollars of deals but have been forced to sit on the loans after panicked investors refused to buy them until the terms are made more palatable.

“There are basically all these huge deals out there and the bankers just want out,” one New York-based investor, who declined to be named, said.

The banks financing the $45 billion (£22.2 billion) buyout of TXU have offered to pay the $1 billion break fee to persuade the private equity firms to drop the deal, although sources say that it would be impossible for them to invoke a MAC, given the strength of TXU’s business.

Last month, Home Depot, the home improvement store chain, was forced to lower the price that it would accept for its wholesale distribution business to $8.5 billion after initially agreeing to sell for $10.3 billion.

In this case, both the private equity firms and the lending banks successfully invoked MAC clauses to show that Home Depot had been materially adversely affected by the collapse of the American sub-prime mortgage sector. The buyout firms were able to pay a lower price and the banks were able to charge more for the debt to finance the acquisition. Sources say that there are similar negotiations going on across several large deals, including the buyout of Sallie Mae, the student loans provider.

Charlie Geffen, the head of private equity at Ashurst, the City law firm, said that it would be difficult for private equity firms to be successful simply because many of the big deals, signed in the M&A boom, did not include MAC clauses.

“Private equity’s appetite for deals has been so strong over the last two years that sellers have been able to dictate terms in the knowledge that there will be plenty of buyers competing for the business. One result of this has been that material adverse change clauses have become virtually nonexistent,” he said.

“In the UK market, short of one side going bankrupt, once the deal is signed it is extremely hard for the buyer to pull out.”

In RBS’s case, sources said that the bank had other ways of getting out of its bid for ABN – for example, if it did not reach its 80 per cent threshold of shareholder acceptances or if regulators blocked the deal. In either instance, RBS could let the deal lapse, then come back with a lower offer weeks later, the sources said.

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