Bianca Jagger is fighting claims that she’s too stingy to pay a reward to a man who found her missing diamond, sapphire and platinum ring. The 67-year-old former wife of Mick Jagger lost the bauble, which she said was valued at about $250,000 while at the 2008 Salzburg Music Festival in Austria. Good Samaritan Reinhard Fingler found the gem, and under Austrian law he would be entitled to 5 percent of the ring’s value as a reward. But Jagger now claims she made a mistake in the value, saying it’s worth only about $23,500 and is instead offering Fingler $1,250. He’s suing her for about $12,500, the amount he feels he’s owed. Lawyers for Fingler yesterday appeared before a judge in Austria, but Jagger, who is in New York this week, has been preaching her side of the story to friends in Manhattan, saying, “The man’s story does not have a ring of truth about it.” A source told us Jagger is “furious she is being portrayed as stingy, like her ex, Mick.” Showing her charitable side, sources said Jagger doled out $5 and a cup of coffee to a homeless man as she strutted along Madison Avenue yesterday morning.