Bayern Munich striker Robert Lewandowski (AFP)
Lewandowski chalking up his 47th goal this season in Tuesday's 1-1 draw at home to Villarreal which saw Bayern knocked out of the Champions League with a 2-1 aggregate defeat in the quarter-finals.
However, Lewandowski, who turns 34 in August, is out of contract next year.
Bayern are taking their time offering him an extension, fuelling reports he may leave with Barcelona touted as one possible destination, which irks Kahn.
"Apparently there's a competition out there: who can tell the biggest nonsense story about Robert Lewandowski?" Kahn fumed after Tuesday's Champions League exit.
Kahn insists the runaway Bundesliga leaders are "not crazy and are now discussing the transfer of a player who scores between 30 and 40 goals with us every season".
Kahn insists Lewandowski won't leave at the end of the current campaign. "We will definitely have Robert with us for another season."
Yet while Kahn tried to insist Bayern are "relaxed" about extending the striker’s deal, the club's lack of action keeps the topic ticking over in the German media.
Lewandowski reacted with surprise last month - "I am hearing this for the first time" - when told sports director Hasan Salihamidzic said Bayern want to extend.
Bayern's top-scorer reportedly earns up to 23 million euros ($25 million) per year in Munich.
"He knows what he has in Bayern - and we also know what we have in him," added Kahn, who said extension talks are "ongoing."
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