Jorge Collazo, from Brooklyn, found the ring in a small white box in a bathroom stall during a Marc Anthony concert at the Brooklyn arena last Saturday.
Collazo, 63, also found a tiny piece of paper with the ring. The note was a declaration of love to "My dear Daniela d'Andrea".
"On this day Feb 15, 2014, I want to give you This Ring as a Simbol (sic) of My Love," the note said.
The last words began "Would you", but the rest were torn off.
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The back of the note read: 'Te quiero mucho', Spanish for 'I love you a lot'.
Collazo, who waited 10 minutes for the guy to rush back to the bathroom to retrieve the ring, is sure it was a proposal, 'New York Daily News' reported.
"If he was rejected, why wouldn't he get his money back and go out with the guys?" Collazo said.