But where does the 24-year-old Dominican Republic-born defender feature in the list of United's costliest signings?
We run through the top ten most expensive players to join Leeds in terms of transfer fees in reverse order - and seven of that top ten are part of Marcelo Bielsa s current first team squad.
Upon joining the club on a four-year deal, Firpo told LUTV: "I'm really happy, I wanted to be here," he said.
"I always wanted to play in the Premier League, which for me is the best league in the world.
"For me to be here in this club, a historic club with really, really good fans. I'm excited to play here.
"This summer for me many clubs were interested in me but when Victor (Orta) spoke with me and showed me the project, I said wow it's interesting, they really want me to play, if I play well."