Officials say they have already shifted 38,000 and it is a formality that the competition's attendance record of 38,137, set by Arsenal in 2007, will be smashed.
United, coached by Travis Binnion, are seeking to win the competition for an 11th time, which would extend their own record.
Tickets for the Old Trafford clash cost £1, with the money going to the club's charity arm.
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