The Central Remedial Clinic (CRC) is insisting it came to an agreement with the Health Service Executive (HSE) in 2009 to allow for the top-up of some salaries.
Members of the CRC board were this morning appearing before the Dáil's Public Accounts Committee (PAC) to answer questions on the top-ups scandal.
Former CRC chief executive Brian Conlon, who resigned his position on Monday, is not attending.
Board Director at the CRC David Martin insisted at the committee that they came to an agreement four years ago to allow for the top-ups, quoting a letter written at the time.
"'We undertake that the CRC as a voluntary organisation will itself fund the excess of salaries concerned as identified by the HSE, with effect from the 1st of June, 2010' - and that's what happened subsequently," Mr Martin said.
"'As and when the management posts concerned fall to be replaced through retirement and resignation, the salary of the incoming replacement will be discussed with the HSE'."
Meanwhile the former chief executive of the CRC, Paul Kiely has refused to describe some of his salary as a 'top up'.
He says his salary was paid in accordance with his contract.
"I was getting a salary from the clinic that would appear to be in excess of what the HSE had 'sanctioned'...because they wrote and said a more appropriate salary in 2009 was a figure that I was already in excess of," he said.
Meanwhile the Social Protection Minister says the former CRC Chief Executive Brian Conlan is 'morally obliged' to attend the PAC.
He resigned on Monday but said he would not be answering questions from TDs.
Minister Joan Burton said she does not believe that is not good enough.
"I think there's a moral issue where he took on - presumably in good faith - the CEO-ship of a very important and significant organisation," she said.
"I think he should give those answers for the sake of the staff of the CRC, the people who use the services and then the people who donate the money."