The former wife of Rolling Stones singer Mick Jagger lost the diamond and platinum ring when it fell off her finger when she was in Austria for the Salzburg music festival in 2008.
Reinhard Fingler, who found the ring, claims Miss Jagger, 67, is making a ‘blatant attempt’ to cheat him out of a proper reward.
Under Austrian law he would be entitled to £8,000, 5 per cent of the £160,000 that she claimed in her police report it was worth.
But she now says she made a mistake about the value and is offering £800.
Mr Fingler has begun a legal action to make her pay £8,000. Miss Jagger has a jeweller’s appraisal to present to a court in Salzburg next Tuesday valuing the ring at £15,000.
Fingler picked the platinum, diamond and sapphire sparkler up in the street.
His attorney Alexander Schuberth said: ‘This is outrageous. I find this claim both bizarre and spurious.
‘Where Jagger got the value of the ring as £160,000 seems to be a mystery as she’s never appeared in court for us to be able to ask her.’
Jagger’s claim that she did not know the value of the ring has raised eyebrows in the elite world of high-end jewellery as her daughter Jade is a gem designer.
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The prestigious Viennese lawfirm of Lansky, Ganzger & partners represents Miss Jagger and did not comment on the alleged lower value of the ring.
Fingler, from Salzburg, initially gave the ring to his daughter thinking it was a trinket, but handed it in eight days later and claimed the reward.
Mick Jagger in full swing at a show in London in 1973 and the couple's daughter, jewellery designer Jade (right)
€1,000 and transfer €9,000 to Amnesty International, an arrangement he declared to be unacceptable.
Judge Friedrich Gruber at the State Court Salzburg will decide next Tuesday whether Austria has the right to hear the case as Ms Jagger has her main residence in London.