Ex-Minneapolis police officer Derek Chauvin’s criminal trial on Wednesday featured testimony from a former convenience store clerk who was handed a counterfeit $20 bill from George Floyd, which ultimately set in motion the man’s encounter with law enforcement.

Christopher Martin testified Wednesday that he watched Floyd’s May 25, 2020, arrest outside Cup Food store with "disbelief" and "guilt."

"If I would’ve just not tooken [sic] the bill, this could’ve been avoided," the 19-year-old Christopher Martin said, joining the burgeoning list of onlookers who have expressed a similar sense of helplessness and lingering guilt over Floyd's death.

Martin said Floyd, 46, handed him what he believed to be a bogus $20 bill while buying cigarettes at the shop.

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Chauvin, 45, faces charges of second- and third-degree murder and manslaughter.

In this image from video, witness Christopher Martin answers questions as Hennepin County Judge Peter Cahill presides Wednesday, March 31, 2021, in the trial of former Minneapolis police Officer Derek Chauvin at the Hennepin County Courthouse in Minneapolis, Minn. Chauvin is charged in the May 25, 2020 death of George Floyd. (Court TV via AP, Pool)

In this image from video, witness Christopher Martin answers questions as Hennepin County Judge Peter Cahill presides Wednesday, March 31, 2021, in the trial of former Minneapolis police Officer Derek Chauvin at the Hennepin County Courthouse in Minneapolis, Minn. Chauvin is charged in the May 25, 2020 death of George Floyd. (Court TV via AP, Pool) ((Court TV via AP, Pool))

Prosecutors used Martin to help detail the sequence of events leading to the arrest, and also played store security footage showing Floyd in Cup Foods for about 10 minutes, adding to the mountain of video documenting what happened that day.

Martin said he immediately believed the $20 that Floyd gave him in exchange for a pack of cigarettes was fake but accepted it, even though store policy was that the amount would be taken out of his paycheck if found to be counterfeit.

Martin said he initially planned to just put the bill on his "tab" but then second-guessed himself and told a manager, who sent Martin outside to ask Floyd to return to the store.

He said a manager asked another employee to call the police after Floyd and a passenger in Floyd's vehicle twice refused to go back into the store to resolve the issue.

Floyd was later arrested outside, where Chauvin was seen in moments captured on video pinning his knee on the man's neck for what prosecutors said was 9 minutes and 29 seconds, as a handcuffed Floyd lay face-down on the pavement. Floyd was later pronounced dead at a hospital.

In this image from video, defense attorney Eric Nelson, left, and defendant former Minneapolis police Officer Derek Chauvin listen as Assistant Minnesota Attorney General Matthew Frank, questions witness Christopher Martin as Hennepin County Judge Peter Cahill presides Wednesday, March 31, 2021, in the trial of Chauvin at the Hennepin County Courthouse in Minneapolis, Minn. (Court TV via AP, Pool)

In this image from video, defense attorney Eric Nelson, left, and defendant former Minneapolis police Officer Derek Chauvin listen as Assistant Minnesota Attorney General Matthew Frank, questions witness Christopher Martin as Hennepin County Judge Peter Cahill presides Wednesday, March 31, 2021, in the trial of Chauvin at the Hennepin County Courthouse in Minneapolis, Minn. (Court TV via AP, Pool) ((Court TV via AP, Pool))

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Inside the store, Martin had asked Floyd if he played baseball, and Floyd said he played football, but it took Floyd some time to respond, so "it would appear that he was high," he said. He also described Floyd as friendly and talkative.

Martin said he left his job at the store after Floyd's death because he no longer felt safe. 

The defense has argued that the now-fired officer did what his training told him to do and that Floyd’s death was not caused by Chauvin's knee on his neck -- as prosecutors contend -- but by a combination of illegal drug use, heart disease, high blood pressure and the adrenaline flowing through his body.

Wednesday was the third day of trial proceedings in the case involving Chauvin. The most serious charge against him carries up to 40 years in prison.

On Tuesday, several witnesses testified that they and other bystanders became upset as they repeatedly begged Chauvin to take his knee off Floyd's neck, but Chauvin refused to ease up, and former officer Tou Thao forced back those on the sidewalk who tried to intervene.

One of those who happened upon the scene, Minneapolis firefighter Genevieve Hansen, wept as she recalled being prevented from using her EMT training to help Floyd.

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"There was a man being killed," said Hansen, who testified in her dress uniform and detailed her emergency training. "I would have been able to provide medical attention to the best of my abilities. And this human was denied that right."

Fox News' Danielle Wallace contributed to this report, as well as The Associated Press.