The 30-year-old impressed during a season-long loan at Craven Cottage last season, earning himself a permanent switch.
Fulham only struck that deal in May, with Cologne sanctioning a sale.
Schalke are, however, hoping to take Riether back to Germany and can offer the experienced full-back UEFA Champions League football.
Fulham are reluctant to discuss a deal, though, and there has been no movement on the transfer front as yet.
Schalke general manager Horst Heldt told kicker-Sportmagazin: "In the case of Sascha Riether we have not made any progress.
"There will be no imminent deal, but we are keeping tabs on the situation and are still hopeful that we will agree a transfer in the end."