Since a flashback is a scene in a story that interrupts the present action to tell about events that happened at an earlier time, there are clearly no flashbacks in O. Henry's "One Thousand Dollars." However, regarding foreshadowing, which is the use of clues that hint at important plot developments that are to follow in a story, there are, indeed, some very subtle hints.
In the exposition of this story, the main character, Young Gillian, admits that he has never been "good at accounts" when he is given his inheritance of one thousand dollars. And, Old Bryson, sighs, lays down his book, and takes off his glasses when Gillian enters the men's club. These two subtle suggestions that Gillian does not value money and that he is unorthodox in his behavior foreshadow the later actions of Gillian. In addition to these two hints, when Gillian stops by the dressing room of Miss Lauriere and her reaction to receiving a necklace is rather blase--
"Oh, just as you say," carrolled Miss Lauriere--
and Gillian leaves, the reader again suspects some rather unorthodox behavior from the main character. He, then, goes to Miss Hayden, the ward of his rich uncle, and again professes his love. But, Miss Hayden tells him she is sorry. Yet, knowing Miss Hayden does not love him, Gillian makes out his check to Miss Hayden and sets out for the law offices of Tolman & Sharp where he must report on what he has done with the money. However, when he hears that if he acts in an unselfish way with the money, his uncle will bequeath $50,000.00 to him, but if he acts otherwise, Miss Hayden will be given the inheritance, Gillian tears up the note. That he tears the note has been foreshadowed by his declaration of love to Miss Hayden; he wants her to have the $50,000.00 because he truly loves her--again an unorthodox act.
Is there a reason that you think there is either of these devices in this story? I do not think that there is anything that really fits in either of these categories in this story.
There is most definitely nothing in the way of flashbacks in this story. All of the action is done in the present. The narrator does not describe any action that happened in the past.
As far as foreshadowing, the only possible foreshadowing that I can see is when Gillian writes the note showing that he loves Miriam. This gives us a hint that he might want to do something nice for her, but it does not really hint at how selflessly he will act at the end of the story.
So I do not really think there is any foreshadowing in this story.