The extra tickets are the result of not as many members taking up their option as was first thought.
And though the ticket office at the north east corner will open at 10am for sales, priority on a first come first served basis will go to those who have ticket stubs from the semi-final first leg against Preston North End at Elland Road.
United chief executive Shaun Harvey explains in the Yorkshire Evening Post: "Individuals can buy up to two tickets maximum as long as they produce the relevant number of stubs from the Preston game.
"We are aware that touts have been collecting the tickets from that game and that is why we are organising things in this way.
"I must emphasise that people are not guaranteed to get a ticket, but they have a good chance of doing so if they follow the criteria.
"There will be no postal applications. It's a last chance for the fans to get their hands on a ticket for the club's biggest game in years."