Commonwealth Bank chief executive Ralph Norris said one of the reasons the bank withdrew from negotiations to buy the local business of the investment bank ABN Amro was that the write-downs announced by ANZ and National Australia Bank had had such a negative impact in international funding markets that the bank thought it was prudent not to go ahead.The bank announced yesterday that it had withdrawn from discussions with RBS over the deal. Norris said: "Most of the investors had come to the view that the Australian banks were clean. The write-downs were a surprise to them. We were told there would be less appetite and the funding cost would be higher. "We could fund it. We could pay for it out of our own resources. But the markets are more difficult and doing a deal like that at this time has more risk associated with it."On July 25 National Australia Bank announced that it was making an additional provision of $830 million against a portfolio of collateralised debt obligations and asset backed securities. That provision was in addition to a $181 million charge the bank had already taken against the $1.2 billion portfolio.And on July 28 ANZ announced provisions for the six months to September would be around $1.2 billion. This was on top of $980 million of credit impairment costs in the March half. ANZ reported that its institutional portfolio was under "significant stress". It also reported that a guarantor of a credit intermediation trade was in financial difficulty.An alternative explanation - that CBA and ABN simply could not agree on price - will have to remain conjecture. The Australian today repeated past reports that the tentative price agreed for ABN's retail stock broking and corporate advisory business was $435 million. RBS said late yesterday it would integrate ABN with its own corporate banking business in Australia.Meanwhile Norris said yesterday that CBA's exposure to CDOs was a net $50 million. "Back in 2004 and 2005 we had exposure to CDOs. We sold out of them in 2006. We looked at going back in the same year but we took the view there was not enough transparency."