Pietersen was spotted at United's training session in Pretoria yesterday, together with George Maluleka. Both players were being put through their paces under the watchful eye of Gavin Hunt.
But Mpengesi says the Cape Town-based defender will appear in Chippa's colours next season, as they still have a one-year option on his contract.
"We spoke to his agent, but we are very disappointed that he went behind our back to take the player to SuperSport United," Mpengesi says.
"We feel undermined by SuperSport United's actions but we are adamant that Pietersen will be playing for Chippa United next season."
However, Pietersen agent's Glyn Binkin sang a different tune when KickOff.com asked him about the player's contract with Chippa.
"I asked Chippa United to show me the contract with the option but they never came back to me," Binkin says. "Roscoe is a SuperSport player, after he signed a contract with them on July 1."