One brake on the planned buyback by Macquarie Group of 10 per cent of its shares, announced at the end of last month, will be the effect on its credit ratings. Two ratings agencies published cautionary opinions on this topic last week.Fitch Ratings said the buyback should have no impact on the long term rating of A and short term rating of F1 in Macquarie debt. Fitch said this view is predicated on any share buyback being funded by capital generated through efficiency initiatives and the potential future issuance of a Basel III-qualifying hybrid capital instrument, and not from the group's existing regulatory surplus capital.Macquarie's presentation to investors the week before last was not clear-cut on this point. And, indeed, running down more than A$3 billion in surplus capital (and helping to lift low returns on equity) was a prime rationale for the plan.Fitch wrote that Macquarie's strong capital position was one of its key rating drivers. It said a material deterioration in current surplus capital, for example through share buybacks, could lead to negative rating action. Moody's Investors Service took a stronger view and placed the A2 (long term) and P-1 (short term) credit ratings for Macquarie on review for possible downgrade.The Moody's announcement emphasised operational features rather than capital management."The review will focus on the outlook for the group's earnings against a backdrop of protracted weakness in the financial markets, and the extent that this trend may be secular as opposed to cyclical," the firm wrote in a media release."The review will also consider the challenges posed by Macquarie Group's expansion, including risk management of an increasingly global and diverse business, as well as greater competition in many global capital markets' business lines."