Mr McMillian also confronted Mr Chauvin after the incident.
Wednesday's testimony before Hennepin County District Judge Peter Cahill was dominated by two witnesses who gave emotional accounts of being at a south Minneapolis intersection seeing Floyd detained by the neck under Chauvin's knee until lapsing into unconsciousness.
Chauvin, who is white, was sacked by the city's police department the day after Floyd, an African American, died in custody.
Floyd says repeatedly that he can't breathe.
Floyd is handcuffed and taken to a squad auto where he becomes increasingly distressed and struggles with the officers who are trying to put him in the back of the vehicle.
"If I would've just not tooken the bill, this could've been avoided", Martin lamented, joining the burgeoning list of witnesses who expressed a sense of helplessness and lingering guilt over Floyd's death.
They took walks in the parks and around the lakes of Minneapolis, which was still new to the Texas-raised Floyd, and ate out a lot: "He was a big man", she said, describing his daily weightlifting, "and it look a lot of energy to keep him going".
Martin said he immediately believed the $20 bill was fake. Darnella - the teenager whose film of Mr Floyd's death sparked global protests - said she "stays up apologising" to him for "not doing more".
"I took it anyways and I was planning to just put it on my tab", he said, meaning that the amount would be taken out of his paycheck.
He said he had considered letting the shop deduct it from his wages instead of confronting Mr Floyd, but then made a decision to tell his manager.
Several onlookers testified Tuesday about their increasing frustration, anger and despair as they begged Chauvin to take his knee off Floyd's neck.
Eric Nelson, Chauvin's defense attorney, claimed in opening arguments that Mr Floyd's death was due to drugs and underlying medical conditions and not asphyxiation.
"He seemed to be having an average Memorial Day, just living his life", Mr Martin said. He told the court Mr Chauvin had used a risky technique called a "blood choke" and was moving his knee back and forth to increase the pressure on Mr Floyd's back and neck.
Mr Martin said he left the store again when he heard "yelling and screaming" outside. "George was motionless, limp".
Martin testified Wednesday that he watched Floyd's arrest outside with "disbelief - and guilt". He explained to prosecutors that he was trying to help Floyd based on his own experiences with law enforcement, arguing he knew that once you are placed in a police vehicle and handcuffed, as Floyd was at the moment, "You're done".
Yesterday, jurors heard testimony from witnesses, and they saw body-camera footage from Minneapolis police officers.
"I feel helpless", McMillan said to describe the way he felt while watching the arrest.
Chauvin, a 19-year veteran of the police force, faces up to 40 years in prison if convicted of the most serious charge-second-degree murder.
"It's one of my favorite stories to tell", Ross said, smiling toward the jury, when asked by a prosecutor how she first met Floyd in August, 2017, at a Salvation Army homeless shelter, where he worked as a security guard. As of now, the trial is expected to last roughly four weeks.