Mr Mullin - according to a report by The Washington Post, which cites officials familiar with the incident - had planned to use the money to charter a helicopter, which he then wanted to use to rescue a family of Americans reportedly stuck in Afghanistan.
According to the officials, Mr Mullin said he planned to fly from Tblisi, Georgia to Dushanbe, the capital of Tajikistan, and told staff he would be arriving just a few hours before he intended to depart. He asked for help from US Ambassador John Mark Pommersheim to move money through the country so he could carry out his rescue.
Mr Pommersheim reportedly said no.
The ambassador was said to be unwilling to skirt Tajikistan's laws regulating the amount of money visitors can bring into the country, especially for a US congressman who was heading to one of the most dangerous spots on the planet.
The officials claimed that Mr Mullin was incensed by the ambassador's response. The congressman reportedly threatened the ambassador and demanded to know the names of the embassy staff who had been handling his request.
It was apparently the second time in as many weeks that Mr Mullin's efforts to enter Afghanistan have been thwarted.
Last week, Mr Mullin flew to Greece, where he asked the US Defense Department for permission to visit Kabul. His request was denied, according to an administration official.
The US State Department is reportedly concerned by Mr Mullin's continued attempts to enter the country without authorisation.
“To say this is extremely dangerous is a massive understatement,” a State Department official told The Washington Post.
Mr Mullin's office issued a statement on the incidents, assuring the public that the congressman "has been and is currently completely safe."
The Independent has reached out to Mr Mullin for comment.
The statement suggests that Mr Mullin is undeterred by the setbacks and may continue his rescue attempts in some manner, claiming his office "will continue to do anything in our power to bring home all Americans from the war zone that President Biden abandoned."
Mr Mullin also made sure to publicise his efforts on Instagram, where he posted a photo of himself from an unknown location alongside the caption: “Have we been helping get Americans out of Afghanistan, yes. Is the mission continuing, yes. Am I missing, no. Did I go dark for a little, yes because it wasn’t safe to be communicating. Am I extremely disappointed in how we (United States) left Americans behind... that would be an understatement.”
He added a hashtag on his Instagram post – “#Ordinarypeopledoingextraordinarythings.”
Mr Mullin was not the only lawmaker to try to insert himself into the evacuation in Afghanistan.
Democratic Congressman Seth Moulton and Republican Congressman Peter Meijer took an unauthorised trip to the country last week, prompting scorn from the Pentagon and State Department. The defense agencies said the trip forced US forces to divert attention and resources to accommodate the lawmakers.
“It’s as moronic as it is selfish,” an official, speaking on condition of anonymity, told The Washington Post.
Mr Meijer gave The Independent’s Eric Garcia his perspective on the situation on Monday.
“One guy texted me after seeing the criticism and said, ‘The only thing you disrupted here was the crummy morale’, because there are a lot of folks who felt ignored or abandoned and just kind of at the end of the world being given this impossible mission,” Mr Meijer said.
Joe Biden said that his administration reached out to Americans still living in Afghanistan 19 times to warn them to leave. According to Secretary of State Antony Blinken, there are fewer than 200 American citizens left in Afghanistan.
US forces evacuated 120,000 people during the operation at Hamid Karzai International Airport in Kabul.
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