Judge Prosser said Peter Nahum had suffered the "blatant lying and devious actions" of dealer Alan Hobart, during the pounds 6.7m sale of View on the Stour by Royal Holloway and Bedford New College in 1993.
The judge said the reason Mr Nahum was never paid his commission by Royal Holloway, part of the University of London, was because of a "veneer of deceit" by Mr Hobart, who represented the eventual buyer of the painting, Sir Graham Kirkham.
Mr Nahum had an oral agreement with the college to find buyers for three paintings which were to be sold to raise money for restoration work. The first, a Turner was sold independently for pounds 11m. Mr Nahum then introduced Sir Graham to the college as a potential buyer of a Gainsborough, Peasants Going to Market, which sold for pounds 3.5m, and he was paid an agreed commission of pounds 100,000.
Even though Mr Hobart had repeatedly told Mr Nahum that his client was not interested in it, Sir Graham went on to buy the Constable. Judge Prosser said Mr Hobart had told the college this was "a completely separate deal", so there was no need to use Mr Nahum.
When Mr Nahum eventually discovered the buyer's identity, he invoiced the college for pounds 196,812 commission, which it refused to pay, saying he had had nothing to do with the sale.
Mr Hobart had explained Sir Graham's change of mind by saying that he had seen the Constable again at the Tate Gallery and because the light was better, had decided to buy it.
But Judge Prosser said he "had no hesitation" in believing Mr Nahum's version of events. "I had a strong feeling throughout that Mr Nahum was an honest man, a straight-dealing and honourable man. I would not rely on Mr Hobart's word for anything in this case."Reuse content