President Trump reportedly implemented a policy allowing him to keep otherwise 'refundable' golf "… membership fees and spend said money on anything he wanted." When Mueller quit one of Trump's golf clubs in 2011, Trump alleged that Mueller was upset by Trump's policy allowing him to keep 'refundable' initiation fees, and thus Mueller was bias against Trump. It seems to me that Trump is running a racketeering scheme while president and should not be allowed to try to intimidate Mueller (by threatening to fire him for not remaining part of Trump's racketeering scheme):
Title: "Why Did Mueller Quit Trump’s Golf Club? Dispute Reportedly Key to President’s Desire to Fire Special Counsel"
http://www.newsweek.com/mueller-quit-trump-golf-club-792562Extract: "Trump argued that Mueller had three conflicts of interest preventing him from leading a fully unbiased investigation, according to the New York Times. The first two conflicts being Mueller’s previous work for a law firm that represented Jared Kushner; the second being Mueller interviewing to return for a second stint as FBI director immediately before being appointed as special counsel in May.
But arguably the most eye-catching alleged conflict of interest involves a purported dispute between Mueller and the Trump National Golf Club in Sterling, Virginia, in October 2011. President Trump claimed Mueller resigned his membership at the club due to the alleged disagreement over fees and believes he still holds a grudge against him over the matter.
Mueller, who was FBI director at the time of the alleged dispute, denied the incident and any potential hard feelings.
…
The hefty fees members are required to pay at Trump golf properties have caused issues in the past.
The one-time initiation fee, which does not include the annual fee, can range anywhere from $14,000 to $450,000 per person, depending on the club location.
And Trump reportedly pockets a large amount of this sum—even as president. The president chose not to sell his businesses after taking office but instead promised to cede control to his sons Donald Jr. and Eric.
Trump is said to have had access to almost $100 million in refundable fees in 2004, these fees coming from just four of his clubs.
The president implemented a policy allowing him to keep the membership fees and spend said money on anything he wanted—a policy that Jay Karen, CEO of the National Golf Course Owners Association, deemed “definitely unusual” in 2017."
Edit: "... per the Washington Post, Mueller merely “sent a letter requesting a dues refund in accordance with normal club practice and never heard back.”"
Obviously, Trump retains initiation fees as sign of fealty, and Mueller betrayed that fealty.