A jury ruled Monday that Michael Oliver, the 11-year-old star of the two “Problem Child” movies, unjustly forced Universal Pictures to renegotiate his contract.The actor may have to return $ 170,000 to the studio. The Superior Court jury found that a signed agreement between Oliver and U was unenforceable as the studio signed it under duress. The studio said the moppet and Dianne Ponce, his manager-mother, threatened to walk off “Problem Child II” on the eve of filming unless his compensation was raised to $ 500,000 from $ 80,000. U has paid Oliver $ 250,000 but insisted it only owed him $ 80,000. An attorney for U said he would ask a judge to order Oliver and Ponce to return the $ 170,000 difference.