The robber stabbed Fox and left with between $600 and $800 from the register, Barr said.
Around 1:15 a.m., police found Fox, 47, unconscious. He was pronounced dead at 1:30 a.m.
Fox was one of four people killed in Philadelphia during a violent four hours Saturday night and Sunday morning. Four other people were wounded in assaults over the same period.
The incidents took place in five neighborhoods scattered across the city, with six of the victims shot or stabbed in just 10 minutes.
Police released few details of T-Barr's incident. Barr, who who was not present at the time of the robbery, filled in what he knew.
Fox was a father of three adult sons and three young daughters, his girlfriend, Debbie DiAntonio, said in a telephone interview.
"He's a good man, and I just want justice. I want the man caught who did this," she said.
On Sunday, a sign on the door said: "T-Barr's is closed today as we mourn the loss of our bartender." Inside, Barr's daughter scraped up blood from the floor behind the bar with a putty knife.
Fox's friend Michael Grimditch, a patron who is a paramedic, stopped by with meatballs, sausage, and spaghetti he had made around 5 a.m.
"Whenever somebody goes like that, you have to eat a little something to refresh the soul," Grimditch said, after handing Barr the dish.
Barr, who is offering a $1,000 reward, said the patrons in the bar didn't call police.
"That's a crime that they . . . watched and didn't do a damn thing about it," he said.
The more he thought about it, the more enraged he became, and a vein popped out in his neck. His daughter urged him to calm down.
T-Barr is a neighborhood beer-and-shot joint, with a narrow space downstairs for customers. Five cameras inside and two outside monitor comings and goings. Some customers have said they don't like that much, Barr said.
Police confiscated surveillance video from the time of the shooting, Barr said.
Grimditch, who also spent some of his Saturday at the bar, recalled seeing Fox earlier in the day and saying goodbye to him about 7 p.m.
"All right, Carrot; I'll see you tomorrow," he remembered saying.
Fox was raised in South Philadelphia. A plumber by trade, he was learning to be an electrician. He was skinny - "like a hanger in a shirt," Grimditch said - but he had an appetite "like a rhinoceros."
"For a regular old South Philly guy, he was full of life," Grimditch said. "He would give his arm if you needed it. He would give you his kidney if you needed it," he said.
Inside, Grimditch and Barr toasted a shot of whiskey to Fox.
"To George," Grimditch said, touching his glass with Barr's.
About 30 minutes later, Barr turned to Grimditch: "I hope we can get over this."
Philadelphia police officials gave sketchy details of the other violent incidents.
At 11:44 p.m. Saturday, officers responded to a shooting in the 600 block of North 38th Street in Mantua, where a 26-year-old man had been shot in the right arm and back. He was taken to Presbyterian Medical Center by a private vehicle and pronounced dead at 12:15 p.m.
Around 1:10 a.m. Sunday, a man who had been fatally shot collapsed right in front of a police car in the first block of North Preston Street in University City. He was taken to Presbyterian Medical Center, where he was pronounced dead at 3:15 a.m.
About 1:20 a.m., four people were shot - one fatally - in a spray of bullets in the 3200 block of Spangler Street in North Philadelphia.
A man, 33, who had been shot in the head, was pronounced dead at the scene. The others were reported in stable condition at area hospitals.
Around 4 a.m., a 45-year-old man was wounded in the buttocks during a possible robbery. The man told police he was walking when he saw three men sitting in a car parked in the 1400 block of Peach Street in the Carroll Park section.
Contact Darran Simon at 856-779-3829 or firstname.lastname@example.org, or reach on Twitter at @darransimon.