Usher is to sack his mother (right) as his manager, it has been claimed.The R'n'B singer is allegedly under pressure from his fiancée, Tameka Foster, to let mom Jonetta go, as the pair reportedly do not get on. A source told the New York Daily News newspaper: "They definitely don't get along. Tameka came to Jonetta's birthday on April 6.They hugged and posed for photos, but it was all for the cameras."Tameka has a hold over Usher.Nobody can figure it out. Some people think he may make Tameka his manager."Executives at Usher's label, Jive records, are reportedly worried about the impending sacking. A second source said: "They think Jonetta's done an incredible job with his career."The rift between Tameka, 38, and Jonetta is said to date back to when Usher's mother urged him to fire his fiancée as his stylist.The source said: "Tameka went on the radio and took some shots at Jonetta."A representative for the 28-year-old singer said: "I don't know of any management changes at this time."Meanwhile, Stuff magazine editors are said to be furious after Usher backed out of hosting the publication's Kentucky Derby tonight. A Stuff insider said: "They originally wanted Justin Timberlake, but someone talked them into going with Usher."The magazine has denied this and said model Taryn Manning will host the event.