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Speaking from Durban, Botha told LiveSport radio this morning he was picked up from his hotel on the Tuesday night before the fight and taken to a "very nice park" where he was asked to lose on purpose.
"My cellphone was confiscated and given to the guy in the car," Botha said. "I was being walked and during this walk I was made an offer of $150,000 to throw the fight.
"I just said, 'no, I can't do this'."
However, Nasser, Williams' controversial manager, has given the latest claim, which came after suggestions the fight was cut from 12 rounds to 10 and an apparent failing of a pre-fight drugs test by Botha, short shrift.
"It's pretty blatant that he's got form in trying to come up with anything to discredit the show," Nasser said. "First it was that he thought it was 12 rounds and then that he knew it was 10 rounds and then that my brother took the drugs test and then okay, someone else took the drugs test and then it was tampered with and ... now this is the latest fantastic tale."
Nasser added of the drugs test claims: "He voluntarily gave his urine, nobody asked him for his urine. He must have been flying high when he did that as well."
Botha said he told a close friend about the alleged offer of a bribe 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'."
The allegation is the latest in an extraordinary series of events around the WBA International heavyweight title fight, won on points by Williams. The fight was originally scheduled for 12 rounds but controversially cut down to 10.
Botha's camp also claimed that as Williams desperately clung on for the win the 10th round was 15 seconds short, but the television coverage of the round has it at the regulation three minutes.
Botha claims he had another drugs test on his arrival back in South Africa and it was clear.
The Sydney Morning Herald reported the 44-year-old submitted a urine sample on Tuesday last week, and the test revealed the presence of phentermine, a banned stimulant.
Botha claimed Williams' camp were scared they would lose, and went to considerable lengths to ensure that didn't happen.
"I wasn't supposed to win this fight right from the start," he said. "This is a 44-year-old veteran kicking it to a 27-year-old."
He added: "Everything was against me - the drug testing, 12 rounds and then down to 10. It's like a race horse training every day for 2000 metres and all of a sudden they cut it down 500 metres and the horse is never going to win because it's not used to it.
"In this fight I was planning to finish strongly in [rounds] 11 and 12.
"Khoder Nasser is very clever. When we started this thing about one year before Sonny Bill got his injuries, the fight was due to be in South Africa and was agreed for a 10-round bout. That was the old contract.
"Later down the road, and when the fight was moved to Australia, I heard it was for the WBA International heavyweight title. I was happy. I didn't need to sign a new contract because I had a contract with my promoter. I was very excited about it."
Now, however, Botha wants a rematch and is meeting with a WBA representative tomorrow to discuss his case.
"I want this fight to be declared a no contest. If this little Sonny Bill, this little fighter that he believes he can be, wants to prove something, stay away from David Tua in the first place because David Tua will knock him out. Fight the White Buffalo and prove yourself as a fighter."