The 29-year-old will also pick up one demerit point for breaching the ICC's disciplinary code, his first such offence. Players can be suspended if they accumulate four demerit points over a two-year period.
"Buttler was found to have breached Article 2.3 of the ICC Code of Conduct for Players and Player Support Personnel, which relates to 'use of an audible obscenity during an international match' following an incident with South Africa all-rounder Vernon Philander," read an ICC statement.
"Buttler admitted the offence and accepted the sanction proposed by Andy Pycroft of the Emirates Elite Panel of ICC Match Referees and, as such, there was no need for a formal hearing."
England won the match by 189 runs on a thrilling fifth day to level the four-match series at 1-1.
The third Test begins on January 16 at Port Elizabeth.