Former Century Radio director Mr Oliver Barry gave over £10,000 in donations to politicians during the 1987 and 1989 election campaigns, he told the Flood tribunal.
Although receipts or letters of acknowledgement were received for every such donation, Mr Barry said he did not ask Mr Ray Burke for a receipt for a separate donation of £35,000.
He was not surprised when a letter from Mr Burke shortly after he gave him the money never referred to the £35,000, he said.
Documents owned by Mr Barry showed he kept a record of the politicians he gave donations to and the amounts he gave them. Mr Patrick Hanratty SC, for the tribunal, said it was "not necessary to refer to the names".
The list showed Mr Barry gave amounts of £5,000 and £2,500 to Fianna Fail members of government at the time. He gave one amount of £1,000 and various amounts of £500, £250 and £100 to Fianna Fail and Fine Gael backbenchers.
When Mr Hanratty asked if one amount for £250 related to a donation to a frontbench Fine Gael politician, Mr Barry replied "no sir", that it was to another Fianna Fail member.
In all cases Mr Barry wrote a letter with the cheques he gave to the politicians and received receipts or letters of acknowledgement.
However, the £35,000 he gave to Mr Burke was paid in cash and no receipt was asked for or given.
"Maybe I didn't think it through properly. I admitted at the outset it was a wrong thing for me to do in hindsight and it has given rise to an awful lot of hardship," he said.
A letter from Mr Burke in July 1989 thanked Mr Barry for "his work during the campaign" and gave him numbers where he could be reached should he need to contact him for "advice or help".
No mention was made of the £35,000 donation.
Mr Hanratty asked Mr Barry if he was surprised no reference was made to the donation. "I don't think it occurred to me at the time," he said.
He added it never occurred to him the donation might have caused Mr Burke difficulties in the future.
Earlier, Mr Barry denied he sought £40,000 from Century Radio in 1991 because he had paid previously to get broadcasting legislation.
Mr James Stafford, also a Century director, had said in a document that £40,000 given to Mr Barry was "paid in connection with getting legislation", Mr Hanratty said.
"It had absolutely nothing to do with legislation," Mr Barry said. The £40,000 was paid to him for work he had done in Century Radio in 1990.