The 21-year-old defender has been poised for a switch to the Ligue 1 champions since Schalke accepted a reported €37 million (£33m/$42m) offer.
PSG coach Thomas Tuchel stated earlier this week that he was unclear as to the stage of negotiations, which appear to still be in their infancy.
Kehrer, who made 28 Bundesliga appearances last season, had agreed a new contract with Schalke prior to PSG's approach.
"Kehrer's transfer to PSG is not yet complete," Heidel told reporters on Wednesday.
"The contracts are still to be signed and he still needs to sort his personal terms."
Schalke head coach Domenico Tedesco is confident his team can cope with the loss of Kehrer should the deal proceed as expected.
"We have a very good squad, a good mix," he said.
"Even without Thilo, we have a total of five options for [the back three], so if we play a back four we only need two centre-backs."
Heidel revealed the club are currently in talks with a left-sided player, although the potential new signing is not expected to be rumoured target Danny Rose.