Botha told South Africa's Rapport newspaper in February he wouldn't mind doing it "somewhere during half-time of a rugby match" and with "one hand tied behind his back".
Williams, 27, has a big fanbase in South Africa and it has been suggested the boxing match could kick off a month-long "sports tour" of the country.
There is also a "50-50 chance" that Williams will play in the 7s Premier League rugby competition in George, South Africa on December 14 and 15, organiser William Strauss told Beeld.
"Quade Cooper has confirmed his participation, and now that there is greater certainty about Sonny Bill's Japanese rugby contract, we still wait for confirmation of his club there," he added.
Williams beat American Clarence Tillman III in Hamilton in February to claim the vacant New Zealand Professional Boxing Association (NZPBA) Heavyweight Championship title.
He has been named in the All Blacks' starting line-up for their Rugby Championship opener against Australia in Sydney tonight.
The rugby star helped the Chiefs win the Super Rugby title earlier this month and has signed a short-term contract with the Panasonic Wild Knights in Japan.
Williams' agent, Khoder Nasser, could not be reached for comment.
-HERALD ONLINE