The attacker has opted against signing a new deal with his current club, fueling speculation that he has set his sights on a transfer elsewhere.
The versatile attacker's current deal runs until the summer of 2015 and a number of sides - including Schalke - are closely monitoring the 25-year-old's situation.
Leverkusen was keen to keep Sam beyond 2015, but the former Hamburg player has rejected the club's final offer.
"Talks between club and player were fair, but there's too much of a difference between what both sides want," Sam's agent, Dirk Hebel, told Express.
Meanwhile, Leverkusen business manager Michael Reschke has insisted that the Champions League hopefuls did whatever they could to tie Sam down.
"We made him an offer that was unfortunately below his expectations. We told him that we have a certain limit and don't want to go any higher," he said. "We have to accept it when he can earn more at another club. He deserves that following his fine performances."
Sam reportedly has a 2.45 million euro buy-out clause in his existing deal, with Schalke ready to pounce at the end of the season.