Lewandowski has recently gone public with his desire to leave the German giants this summer despite still having one year remaining on his contract.
The Poland international has spent eight years in Bavaria, having joined Bayern Munich from Borussia Dortmund on a free transfer in the summer of 2014 and has won eight Bundesliga titles in his time at the club.
The 33-year-old's list of honours at the Allianz Arena also include one UEFA Champions League, three DFB-Pokals, one UEFA Super Cup and one FIFA Club World Cup.
Lewandowski has strongly been linked with a move to Barcelona but Hainer is convinced Bayern Munich have what it takes to retain their talisman.
"A contract is a contract! Where are we going if a player can end a contract prematurely while we as a club would have to pay him in full up to the last day of the term?," Hainer said in an interview with Bild.
"I was a bit surprised that Robert chose to go public, I wouldn't have done it in his place.
"Bayern is a club that treats its players very well, that does everything to enable them to perform at their best. And I have to say, appreciation is not a one-way street!"
"We're in the fortunate position that we don't have any financial difficulties," he added. "We want to have the best players and Robert is one of the best.
"That's why I'm confident that he'll be with us again next season. Robert is a full professional, he wants to play football."
