What is the dramatic irony in "One Thousand Dollars"?

The dramatic irony of O. Henry's short story "One Thousand Dollars" involves Gillian's interpretation of the will's stipulation that he must render "an account of the manner of expenditure of this $1000" that is left him by his late uncle.
When Young Gillian is bequeathed one thousand dollars by his wealthy uncle, who has supported him throughout his entire life, he takes a rather flippant attitude about having to make an account of its expenditure. For instance, he goes to the men's club and asks Old Bryson, who dislikes him, "Now what can a man possibly do with a thousand dollars?" With disinterest, Bryson offers a sarcastic suggestion after moralizing on what this money could buy. Bryson's suggestion certainly implies foolishness:

"Why, Bobby Gillian, there's only one logical thing you could do. You can go buy Miss Lotta Lauriere a diamond pendant with the money, and then take yourself off to Idaho and inflict your presence upon a ranch. I advise a sheep ranch, as I have a particular dislike for sheep." 

Facetiously, young Gillian thanks him, adding "I thought I could depend upon you." When he does visit this actress at the Columbine Theatre, Miss Lauriere is not interested in such a bauble because a friend has received a $22,000. gift. Rejected, Gillian asks people at random what they would do with a thousand dollars, hoping for an idea he can use to rid himself of this sum. Finally, he decides to give it to the woman he loves, Miss Hayden, who worked for his uncle.
After he arrives at the mansion of his uncle, Gillian pretends that the lawyers have found that his uncle "loosened up a little on second thought" and bequeathed another thousand dollars to Miss Hayden. When he hands her the money, she cries, "Oh!" but when he declares his love for her, she merely says, "I am sorry." Gillian departs after writing down how he spent the $1000 and sealed it in an envelope to give to the attorneys in charge of his uncle's estate.
After he arrives at Tolman & Sharp's offices, Gillian informs them, "I have expended the thousand dollars." Then, Mr. Tolman pulls from an immense safe a large envelope sealed with wax. Mr. Tolman tells Gillian that if his disposal of the money has been "prudent," he is to be given $50,000. If, however, he has spent the money foolishly, the money goes to Miss Hayden. 
*Here lies the dramatic irony: Gillian has meant to be rid of the $1000 by giving the money away in any manner that he can. When his first efforts to dispense with it fail, he gives it to Miss Hayden by pretending that it is a reward for her service to his uncle. This act is an example of dramatic irony because Gilliam has believed his action of giving his money to someone like Miss Hayden will be considered foolish by his uncle when, in truth, it qualifies him for an additional inheritance of $50,000 as provided for in the codicil, which is kept hidden until the $1000 is spent.
Having returned to the law offices, Gillian has the codicil of his uncle's will explained to him, and he now realizes that his charity will be judged as an altruistic and mature act, rather than a foolish one, thus entitling him to the $50,000. So, he swiftly grabs the envelope before the older gentlemen can read what he has written inside, saying that he has lost the $1000 in gambling, thereby disqualifying himself from the larger inheritance.Pleased that his beloved Miss Hayden will now receive this money, Gillian walks away whistling in delight.