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The 38-year-old Nongqayi, a former WBF and IBF junior-bantamweight champion, quit the fistic sport after failing to agree on a purse with Branco Sports Productions boss Branco Milekonvic.
Nongqayi accused Milenkovic of indirectly forcing him to quit boxing, which has been his only source of income since making his professional debut against Bongani Thwala at Carnival City on September 14 2002.
But Milenkovic says he does not think he did anything wrong by offering Nongqayi R20000 for an eight-rounder against Oscar Chauke on the undercard of the IBF minimumweight title contest between current champion Nkosinathi "Mabhere" Joyi and Japanese challenger Katsunari Takayama.
"I'm retiring from boxing because it has become clear to me that my promoter doesn't value me anymore," Nongqayi said. "It's really unbearable when a promoter indirectly tells you that you are no longer wanted by offering you less money even though he got millions from the government.
"Branco and I agreed that I would fight in a 10-rounder on the undercard of Joyi's fight next week. However, I was shocked when he sent a contract last week, stating that I was going to fight in an eight-rounder and I would get R20000.
"I refused to take the fight because the purse was an insult to me. I'm retiring now because I can no longer tolerate being treated this way."
Responding to Nongqayi's claims, Milenkovic said: "When a rated boxer fights for a South African title over 12 rounds, the purse is normally between R12000 and R15000. I don't think there is anything wrong with fighting only eight rounds for R20000. Simphiwe's name doesn't even appear on the SA ratings."
Milenkovic added that Nongqayi's decision to quit boxing may not be wrong as he last won a fight in 2009 and was stopped in his last two fights.
The father of three retired after 16 victories (six knockouts), a draw and two losses.
The 2009 Boxer of the Year was 29 years old when he made his professional debut in 2002 after spending 17 years in the amateur ranks where he registered more than 500 fights.
He shocked the boxing world when he defeated Jorge Arce in Mexico to lift the IBF junior-bantamweight crown. That victory came just seven months after he had beaten Jorge's younger brother, Francisco, in Mexico.
"I'm the only boxer in this country who went to Mexico and beat the Arce brothers," he boasted.
Nongqayi was, however, unlucky in his last two visits to Mexico as he lost by stoppages.
He surrendered his IBF belt via a sixth-round stoppage to Juan Alberto Rosas on July 31 2010 and was stopped in the fourth round by Jorge when they fought for the WBO junior-featherweight title last September.
"I'm going to apply for a trainer's licence so that I can impart my knowledge to up-and-coming boxers," he said.