One of the captors recognised him from his school days and let him flee, the man, Alkheir Ismail, said in a video interview conducted by a local journalist known to Reuters in the nearby town of Tawila in the country's western Darfur region.
"He told them, 'Don't kill him,'" Ismail said of the events over the weekend.
"Even after they killed everyone else - my friends and everyone else."
He said he had been bringing food to relatives still in the city when it was captured by the Rapid Support Forces on Sunday - and, like the other detainees, was unarmed. Reuters could not immediately verify his account due to the conflict but has verified earlier material obtained from the journalist.
Ismail was one of four such witnesses and six aid workers interviewed by Reuters who also said people fleeing al-Fashir had been gathered in nearby villages and men separated from women and removed. In an earlier account, one of the witnesses said gunshots then rang out.
Activists and analysts have long warned of revenge killings based on ethnicity by the paramilitary Rapid Support Forces (RSF) if they seized al-Fashir - the last stronghold of the Sudanese military in Darfur.
The UN human rights office shared other accounts on Friday, estimating hundreds of civilians and unarmed fighters may have been executed. Such killings are considered war crimes.
The RSF, whose victory in al-Fashir marks a milestone in Sudan's two-and-a-half-year civil war, has denied such abuses - saying the accounts have been manufactured by its enemies and making counter-accusations against them.
Reuters has verified at least three videos posted on social media showing men in RSF uniforms shooting unarmed captives and a dozen more showing clusters of bodies after apparent shootings.
A high-level RSF commander called the accounts "media exaggeration" by the army and its allied fighters "to cover up for their defeat and loss of al-Fashir."
The RSF's leadership had ordered investigations into any violations by RSF individuals and several had been arrested, he said, adding that the RSF had helped people leave the city and called on aid organisations to assist those who remained.
He said soldiers and fighters pretending to be civilians had been taken away for interrogation.
"There were no killings as has been claimed," the commander told Reuters in response to a request for comment.
The RSF's capture of al-Fashir entrenches the geographical division of a country already reduced by the independence of South Sudan in 2011 after decades of civil war.
In a speech on Wednesday night, RSF head Mohamed Hamdan Dagalo called on his fighters to protect civilians and said violations will be prosecuted. He appeared to acknowledge reports of detentions by ordering the release of detainees.
