Volunteers at the YMCA in Birkenhead, Merseyside, had learned about the Fab Four playing a gig at the branch in 1962 - weeks before they stormed the US.
They delved into the archives to look for proof that John, Paul, George and Ringo - his first gig with the band - performed in the former gym.
But no paperwork or old photographs appeared to exist.
Then one day while walking through a corridor of the hostel Chief Executive Nigel Hughes spotted a yellowed sheet of paper in a bin.
On closer inspection Mr Hughes realised he had found an invoice dated 8th September 1962 for a performance by "Beatles Dance Band".
The document reads: "Received the sum of thirty pounds (£30.00) being payment for appearance 8:30 to 9:30pm. Saturday, 8th September 1962."
It is signed by the late Neil Aspinall who was the Beatles road manager in the days before Beatlemania and their 10 year domination of the pop charts.
The date the band performed in Birkenhead is significant because it is between recording dates at Abbey Road studio in London for the hit 'Love Me Do'.
The song was released on 5th October 1962 - less than a month after the Merseyside gig - and the band hit the big time.
YMCA boss Mr Hughes said: "We have approached an auction house who are very excited about it and asked to see a copy for valuation.
"It's very rare to have a pre-fame item of this kind so I'm thrilled it has been found."
Beatles afficianado and musician Dean Johnson said: "It's incredible to think that this rare document could have been thrown away.
"But luckily it was saved and adds to the pre-fame history of the band and it could fetch thousands at auction."
"What is interesting is they played for an hour because when they became the most famous band in the world they played for about twenty minutes.
"The YMCA gig would have also been one of Ringo's first, if not the first, time he played with the band following Pete Best's sacking.
"I'd like to know what kind of reception he got from their fans."
Auction house Bonham's said their film and music experts were looking at the Beatles invoice but no valuation has been performed.