If someone is found guilty in a magistrate court can they ask for a retry before the sentence is passed?

In Magistrate Court the sentence is generally handed down immediately. You can then appeal. Unlike appeals for convictions in a District Court (or Superior or Supreme Court depending on the state's terminology) where you can only appeal on issues of law, appeals from Magistrate Court are "trial de novo" (meaning new trial)--this is because they are not "courts of record" (there is no stenographer taking everything down) . These appeals are heard at the next level of court, such as a County Court at Law (depending on the state). You will have to post a bond. In Texas the minimum appeal bond is $300 even if the fine was only $50.