A Manhattan oncologist who charged a dying patient $50,000 for an experimental cancer treatment he never performed hasn’t returned the dough, according to a lawsuit.

Cheryl Stoebenau, 69, was being treated for ovarian cancer in 2015 when her doctor, William Grace, told her about a treatment that wasn’t FDA-approved, but “that potentially could save her life,” according to court papers.

Stoebenau would have to pay the $50,000 cost up front because insurance wouldn’t cover it, said the doctor, who also told her she needed to be healthier to receive the treatment.

Stoebenau, a marketing guru who once worked on licensing deals for Cabbage Patch Kids, ran CAS Marketing out of a West End Avenue space. She wrote a $50,000 check from CAS Marketing to the doc, but her condition quickly deteriorated, despite a stint at Lenox Hill Hospital intended to strengthen her.

She died in September 2015, but Grace never returned the cash, according to the Manhattan Supreme Court lawsuit filed against him by CAS Marketing.

Grace told The Post he couldn’t return the drugs once they were purchased, and asked Stoebenau what he should do once it became clear her health was failing. He offered to give her the drugs, Opdivo & Yervoy, anyway, but claims Stoebenau declined.

“I asked what to do, she said, ‘You probably have someone who can’t afford it and doesn’t have the resources that I have, why don’t you give it to them,’” Grace told The Post.

“I went strictly with Mrs. Stoebenau’s wishes. … It’s not like I’m trying to scam people in any way,” he said, adding that he treated a patient with the drugs meant for Stoebenau.

“Dr. Grace was paid $50,000 to administer an experimental cancer treatment, but the patient died before she could receive it. He did not earn this money and should refund it,” a CAS attorney, Mark Pincus, said.