Diomandé, 19, arrived at RB Leipzig last summer from Leganés for €20m and has developed rapidly into one of the most coveted young wingers in European football — attracting attention from PSG and Liverpool among others.
Schäfer addressed the speculation directly in comments to Bild.
"Our intention is clear: Yan Diomandé will play for RB Leipzig next year," Schäfer said. "And we will not budge on that."
"If Yan continues on this trajectory, the moment will come to allow him to take a new step — but not this year. In principle, we have a four-year contract and we hold the cards.
"These current developments are above all a recognition of his performances. Nothing more, nothing less. And if we don't find a solution, then so be it — he has four more years on his contract. I repeat: Yan Diomandé will remain a Leipzig player."
Diomandé is currently at the World Cup with Ivory Coast, who face Norway in the round of 32 on Tuesday, and deflected all questions about his future at a press conference the day before.
"My dream is to play for my country. I no longer have Instagram, I don't have TikTok. I can't see anything," he said.
"I have an agent who will take care of the rest. For me, the most important thing is to be focused on the World Cup and make history."
Pressed again at the end of the session, he was equally firm.
"You are informing me. I don't have internet, I don't have Instagram, I don't have TikTok. It's my agent who is talking to the club — I have no information. I don't want to talk about that, even with my agent, because this is a chance to play at a World Cup. It happens every four years, so you have to make the most of it. I want to make history with my country and give my best."
Whether PSG's desire to sign him before the window closes forces Leipzig to revisit their position remains to be seen — but Schäfer's comments leave little room for ambiguity.