Speaking to the WAZ, Tillmann confirmed the club had built a departure into its financial planning but insisted any deal would have to meet Schalke's valuation.
"A sale is currently included in our planned figures. Let's take the example that's often mentioned in the media, Moussa Sylla: if someone comes along and offers three million euros, we say no," he said.
Tillmann declined to name a specific asking price for the striker, long considered Schalke's most valuable saleable asset, but made clear the club's expectations remain well above recent reported figures.
"I would never name a concrete number. But three million euros would definitely be too little. I'm convinced we will reach break-even and meet the equity requirement. The only question is: how do we get there? Plan A is to reach the necessary equity through transfer proceeds. But we also have a Plan B, C and D."
Schalke's finances remain under significant strain, with the club sitting on negative equity of €96.6m that must be addressed under DFL licensing requirements. Sylla, 26, has previously come close to a departure, with a move to New York City FC collapsing at the last minute in January after the two clubs had reportedly agreed a fee worth up to €7m with bonuses.
The Mali international's contract runs until 2028, and while Schalke are said to have lowered their asking price to around €4m in recent days, Tillmann's comments suggest the club remains determined to secure a fair valuation rather than accept a bargain offer simply to boost the balance sheet before Tuesday's deadline.
Schalke had previously been braced for the possibility of needing to sell several established squad members to meet their financial targets, but recent contract extensions for Vitalie Becker, Soufiane El-Faouzi and Mertcan Ayhan have taken those names off the table, leaving Sylla as effectively the last significant asset the club could realistically cash in on before the window shuts.
Sylla has continued to perform well on the pitch regardless of the speculation, contributing a goal and an assist in Schalke's DFB-Pokal win over Hallescher FC, results that have only strengthened the club's resolve not to let him go for a knockdown fee.