Blatter was grilled by his executive committee on Wednesday over ISMM/ISL's bankruptcy after European soccer's ruling body UEFA put down 25 questions it demanded Blatter answer.
On Friday, UEFA chief executive Gerhard Aigner expressed doubt over the figures in an interview on German radio. Blatter says ISMM/ISL's bankruptcy will cost his organisation 100 million Swiss francs ($56.75m) at the most.
"The calculations up until now want to blur some things. I think the figures are higher than announced so far," Aigner said.
"One can also get into difficulties with a lot of money. Because of this, one must work in a budgetary way, lead a budget, carefully manage finances and carry out good controls. That has not been the case up until now."
Hours later, FIFA issued a statement answering Aigner: "FIFA president Joseph S Blatter stresses that both the FIFA Finance Committee and the FIFA Executive Committee (including seven European representatives) have approved these figures.
"If any questions remain unanswered, FIFA will not debate them in public, but during the next meeting of the FIFA Executive Committee, to be held in Buenos Aires on July 5, 2001."
Blatter's statement to the Executive Committee on Wednesday partially satisfied UEFA officials but they have asked him to provide written answers to several questions.
The July congress in Buenos Aires may see a vote of no confidence in Blatter if delegates are not satisfied with his explanation of the ISMM/ISL debacle.
FIFA president Sepp Blatter defended himself on Friday against claims he had underestimated the losses suffered by world soccer's ruling body from the collapse of its marketing partner ISMM/ISL.
