Eric Lefkofsky, chairman and co-founder of Groupon

Eric Lefkofsky, chairman and co-founder of Groupon

Matthew Staver/Bloomberg

Eric Lefkofsky, chairman and co-founder of Groupon Inc., may parlay a $1 million check to co-founder Andrew Mason four years ago into a stake worth $2.3 billion when the daily-deal site sells shares to the public next month.