Head coach Jason Garrett told reporters Martin was at the team facility on Tuesday but missed practice due to the talks and will continue to do so.
"There's a business side that both sides are working through," Garrett said.
Earlier Wednesday, Todd Archer of ESPN first reported on Martin's absence.
The Cowboys selected Martin with the 16th overall pick in the 2014 draft, and he's since developed into one of the league's best interior linemen.
Martin has earned four straight Pro Bowl selections and two First Team All-Pro nods. He's also been a model of consistency, appearing in all 64 of the team's regular-season games since his debut.
Martin said last week he was focused on preparation for next season, not his contract status, per Patrik Walker of 247Sports.
"I think it's been better after we've gotten into the facility with all the guys," he said. "I really don't think about it now, because I'm with those guys working out and kind of back to normal. We'll see. Hopefully it gets done. We'll see where it goes."
Martin is set to enter the final year of his rookie contract, which will pay him $9.3 million in 2018.
He'd be eligible to become an unrestricted free agent at season's end if the sides don't come to terms on an extension and the Cowboys don't use the franchise tag on him.