Correctional Services Minister Ngconde Balfour has denied promising a special remission to a prisoner in Malmesbury.
Xolani Mahambehlala is serving 18 years for car theft and escape from custody.
He claimed Balfour had offered him a sentence remission of two years for his role in exposing corruption at the Malmesbury Correctional Centre in 2005.
Balfour's spokesman Luphumzo Kebeni said: "The claims are false and baseless because the minister has no powers to order the correctional supervision and parole board to reduce an offender's sentence."
The reaction follows a weekend report that Mahambehlala was taking Balfour to court in an effort to have his sentence reduced by at least two years.
According to Kebeni, the prisoner was assigned by police to secretly film correctional services officials involved in corrupt activities that included smuggling illegal items into the prison.
"It should be borne in mind that this was an operation approved by the department and the directorate of public prosecutions and jointly conducted with the SAPS.
"This was not his own initiative at all and he cannot hold the minister to ransom," he said.