Mr Robinson bought the access strip of land for £5
Northern Ireland First Minister Peter Robinson, and his wife Iris, bought a valuable bit of land from a developer for just £5, BBC Newsline has learned.
The land deal enabled the Robinsons to sell part of their back garden for nearly £460,000. They could now be liable for thousand of pounds in taxes.
They sold the land for £5 to a different developer, allowing the deal for their garden to go through.
The DUP has accused the BBC of a smear campaign against Mr Robinson.
The Robinsons acquired the strip of land 50 metres from the boundary of their back garden at Gransha Road in Belfast from a property developer friend, the late Fred Fraser.
The land was the gateway to a proposed housing scheme, covering the Robinsons' garden and others.
The strip in question at Fort Road was left over from one of Mr Fraser's earlier developments.
He knew it was valuable because BBC Newsline talked to a builder who said he tried to buy it, but dropped the idea when Mr Fraser indicated the cost would be a percentage cut of every new house.
Fraser Houses at first said the strip had been surplus to requirements.
Later they said the decision to sell so cheaply could not be explained any further because it had been Mr Fraser's decision and his alone. Mr Fraser died in August 2008.
Mr Robinson approached Mr Fraser at the request of another developer who would later purchase part of their back garden for £459,000.
On the same day they sold the garden plot, Mr and Mrs Robinson sold that developer the access strip for the same £5 sum they had paid Mr Fraser 18 months earlier.
The developer's company, Bloomfield Developments, registered in Bangor, had been working to put together a deal involving the gardens since 2003.
Desmond McLaughlin of Bloomfield Developments responded to questions via a letter from his solicitor.
The letter said Mr McLaughlin had no contact with developer Fred Fraser over the strip but was aware that Mr Fraser was a friend of the Robinsons.
It said that when speaking to the couple about their property, Mr McLaughlin asked if Mr Robinson would see if the access strip could be acquired.
Mr McLaughlin would not be drawn on how the price was agreed but did say, that in terms of the overall deal, he paid a "proper" price to each seller after negotiations.
A professional valuation carried out for the BBC put the strip's value at upwards of £75,000 when the Robinsons obtained it, and at least £220,000 when they sold it.
Although they made no profit, tax experts say Customs and Revenue could use market values on the sale instead of the £5 they bought it and sold it for.
This potentially exposes the Robinsons to a tax bill of thousands of pounds.
The BBC twice asked Mr Robinson if they declared or paid any taxes, but he has not responded.
Another declaration Mr Robinson needed to consider involved Westminster.
In 2005, land or property holdings totalling more than £57,000 had to be noted on the MPs' Register of Interests.
At that time Iris Robinson did not declare anything. Mr Robinson had registered ownership of a constituency office, so anything of further value should have been added but at no time did he ever declare his co-ownership of the strip.
He also has refused to say why he did not declare a potential conflict of interest when the planning application for the scheme went before Castlereagh Borough Council for consideration.
In 2007, Bloomfield Developments sold the garden plots on to another developer for £3.6m.
The housing market crashed before anything was built.
However, the Robinsons are hundreds of thousands of pounds richer from a deal made possible by Fred Fraser's £5 access strip.
BBC Newsline asked the Robinsons for an explanation of the issues raised in this report on 1 March and again on 24 March. A reminder was sent on Monday.
On Tuesday, the DUP described the BBC Newsline report as an example of the "ongoing BBC smear campaign" against its leader.
In a statement on Tuesday, the party said Mr Robinson sold his land at the market-value and that there are no tax implications.
The DUP said the sliver of land was bought for £5 and sold for £5, a fact that has never been hidden and Mr Robinson derived no financial benefit whatsoever from the transaction.
The DUP statement said Mr Robinson has handed the issues over to his legal team to be dealt with.