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Botha, speaking from Durban, told Radio LiveSport that he was picked up from his hotel on the Tuesday night before the fight and taken to a park where he was asked to lose on purpose, he alleges.
"My cellphone was confiscated and given to the guy in the car and I was being walked. During this walk I was made an offer of $150,000 to throw the fight," Botha said.
"I just said 'No I can't do it'."
Why Botha has left it until five days after the fight to make the serious claim is unclear.
He claims he told a close friend about the alleged offer on his return to the hotel.
"As I came into the hotel he could clearly see that I am very upset. I tell him I need to see him and talk about this."
"He said 'Fran if you ever think about doing this I'm going to get on a plane and go back to South Africa because I don't want to be part of this'.
I told him straight 'Listen I'm not doing this. I'm a Christian and I cannot do this'.
"Everything there, I wasn't supposed to win. Everyone was against me, then the drugs testing, then being 12 rounds...then cut down to 10."
Botha, who reportedly failed a drugs test on the eve of the fight claims he had another test on his arrival back in South Africa and passed.
According to reports the 44-year-old submitted a urine sample on Tuesday last week, the Sydney Morning Herald said, and the test revealed the presence of phentermine, a banned stimulant.
"I did my drugs test and the results are all clear."
Botha claims the drug testing process the week before the fight was done by a relation of Khoder Nasser.
"They called me on the Tuesday, I did the drug testing in the morning and the way I went and did the drugs testing it was suspicious probably because it was a small room. I did the urine and I've done other drugs test in my career and you usually do sample A and sample B. It gets sealed.... the same with the blood. This wasn't done I only did one test and the blood was not regulated," Botha said,
"As far as I know only the Boxing Commission of Australia can do this test and it wasn't done by the Boxing Commission of Australia.
"This was done by Khoder Nasser's brother or his nephew, somebody like that - a relation to him."
SBW-Botha controversy timeline
Last Tuesday morning - Botha takes drugs test.
Last Tuesday evening - Botha offered money to throw fight, he alleges.
Thursday - At the weigh-in Williams says Botha will do "12 rounds pretty easy".
Friday - Williams wins fight by judges decision. The bout lasts 10 rounds, when many expected a 12-round fight.
Saturday - Williams tweets 'We both knew it was a ten -rounder'. TAB refunds all 'pick the round' bets on the fight.
Monday - Sydney Morning Herald reports that Botha's drug test revealed the presence of phentermine, a banned stimulant.
Today - Botha alleges to Radio LiveSport he was offered money to throw the fight.