Bryson suggests that he buy Miss Lotta Lauriere, an actress, a diamond necklace.

2. When Gillian asks her if she would like a pendant for a thousand dollars, Miss Lauriere alludes to another actress she knows who has received a necklace that is worth much more. Defeated, Gillian leaves, but he asks the cab driver what he would do with $1000. When he does not like the man's answer, Gillian then asks a blind man how he would spend the money. The man shows Gillian a bank deposit book with more than a thousand dollars in entries; Gillian returns the book and re-enters the cab.

He then goes back to the law offices and inquires if Miss Hayden has been left anything besides a ring and $10. Lawyer Tolman replies in the negative. So, Gillian has the cab take him to the house of his dead uncle where Miss Hayden is seated, writing letters in the library. Gillian tells
her that the old lawyers have found a codicil to the will and she is to receive one thousand dollars. Further, he tells her that he was driving this way and lawyer Tolman asked him to bring it to her. Blanching, Miss Hayden can only utter, "Oh!" and repeat "Oh!" Gillian declares his love for Miss Hayden, but she responds, "I am sorry," and takes the money.

Going into the next room, Gillian writes out the account of his expenditure:

Paid by the black sheep, Robert Gillian, $1000 on account of the eternal happiness, owed by Heaven to the best and dearest woman on earth.

Slipping this into an envelope, Gillian departs.

3. Gillian returns to the offices of Tolman & Sharp with his written accounting of how he spent the $1000. He tosses the white envelope upon the table before Mr. Tolman, saying, "You will find there a memorandum, sir, of the modus operandi [method] of the vanishing of the dollars."

Without looking in the envelope, Mr. Tolman calls his partner and together they explore the inside of a huge safe. Finally, they bring out a very large envelope sealed with wax; then they explain that there is a codicil to his uncle's will. When Bobby Gillian has spent his $1000, then this document is to be read. The lawyer reads,

"If your disposal of the money in question has been prudent, wise, or unselfish, it is in our power to hand you over bonds to the value of $50,000, which have been placed in our hands for this purpose....But, if....you have used this money as you have used money in the past—I quote Mr. Gillian—in reprehensible dissipation among disreputable associates,—the $50,000 is to be paid to Miriam Hayden....I will examine your account in regard to the $1000....I hope you will repose confidence in our decision."

As Mr. Tolman reaches for his envelope, Bobby Gillian grabs it first; he tears it into strips and places it into his pocket. He tells the old gentlemen they need not bother to read his account of his itemized bets. "I bet the thousand dollars at the races. Good-day to you, gentlemen." Mr. Tolman and Mr. Sharp shake their heads in disapproval as they watch Gillian depart, whistling happily in the hallway as he goes toward the elevator.

O. Henry's short story "One Thousand Dollars" tells the tale of Bobby Gillian, a carefree man who one day learns that he has inherited a thousand dollars from his uncle—with one stipulation. He must provide an accounting of how he spent the money to the lawyers who are the executors of the will. Gillian complains that it is such an "awkward" amount and seeks out advice on how to spend it. In the past it seems, Gillian had spent money recklessly on women and gambling. When no one supplies him with a suitable way to dispose of the money, he decides to give it away to Miss Hayden, his uncle's ward, a woman he just happens to be in love with. Instead of admitting that he is giving her the money, he uses the excuse that the lawyers found an amendment to the will which left her a thousand dollars. He then professes his love for her, but all she can say is "I am sorry." He then writes out an accounting for the money writing,

"Paid by the black sheep, Robert Gillian, $1,000 on account of the eternal happiness, owed by Heaven to the best and dearest woman on earth."

Gillian then delivers the accounting to the lawyers who speak of a "codicil" to the will which reveals that if Gillian had actually spent the money in a "prudent, wise or unselfish" manner, instead of the way he "used money in the past," he would receive $50,000. If he had spent the money foolishly the money would go to Miss Hayden. After this is revealed, Gillian quickly takes up the envelope with the accounting and informs the lawyers that it doesn't matter because he lost the money gambling.

The ending is ironic for two reasons. Gillian had actually done what his uncle wanted, but ultimately sacrifices the money for Miss Hayden by not telling the lawyers how he used the money. It's also ironic that he would give all the money away to a woman who didn't even love him.   

I think that there is a little bit of irony in the end of this story.  Specifically, I think that the attitudes of Tolman and Sharp, the lawyers, toward Gillian are ironic.  This is because they completely misunderstand what is going on.

Toland and Sharp think that Gillian is just some playboy wastrel who has frittered away his inheritance just the way that his uncle feared that he would.  They think that he has not grown up and has not shown any sort of responsible attitude.

What is ironic is that he really has grown up.  He has acted in a completely unselfish way by giving up his inheritance so Miriam Hayden can have a good life.

So the lawyers have a completely mistaken impression of what has happened and that is somewhat ironic.