Kehrer, 21, started 27 Bundesliga matches for Schalke last term after breaking into the first team during the 2016-17 campaign.
Heidel has revealed that Schalke were desperate to keep hold of the defender, but could not reject a proposal which is believed to be worth in the region of £33m to the German outfit.
"Without an offer of the dimension that PSG has given us, Thilo would have certainly extended," Heidel told reporters. "We have basically agreed on a contract with Paris. The transfer will have an economic significance that Schalke 04 can not refuse."
PSG will open their 2018-19 Ligue 1 campaign at home to Caen on Sunday night.