Ken Campbell, a close friend of her husband, First Minster Peter Robinson, said he contributed between 4,000 and 5,000.
But even though he confirmed he also wrote a cheque for 25,000 to help Mrs Robinson's secret lover set up a business, he categorically denied making any personal donations to the Robinsons.
A spokesman for Mr Campbell said: "He did make a small contribution to the DUP several years ago. He never made a personal donation to Iris or Peter Robinson."
Mrs Robinson secured 50,000 from Mr Campbell and another wealthy developer, Fred Fraser, who died last year, to finance her lover Kirk McCambley's new restaurant on the banks of the River Lagan, south Belfast.
She kept 5,000 for herself and then demanded Mr McCambley pay the money back after their affair ended.
Lawyers for Mr Campbell said the money – a cheque written out in his name – had been loaned to Mr McCambley on an interest-free basis and 20,000 had been repaid. The remaining 5,000 was outstanding.
Details of Mr Campbell's contribution to the DUP emerged after Arlene Foster began her first day as the temporary first minister, as Mr Robinson stands down from the post for six weeks to care for his wife, who is undergoing acute psychiatric treatment at their home in east Belfast.
The couple are to be investigated by the assembly's committee on standards and privileges after Mrs Robinson failed to declare her interest. Mr Robinson said he was confident he would be cleared of any wrongdoing.
The inquiry is being headed by Tom Frawley, the Northern Ireland ombudsman.