In a rare marketing ploy, the No. 2 U.S. movie theater chain, AMC Entertainment, is offering a money-back guarantee for boxing picture "Cinderella Man," hoping to boost interest in the struggling film amid a record box-office slump. Advertisements offering on-the-spot refunds to AMC patrons unhappy with the film began running June 24 in newspapers and on the exhibitor's Web site (www.amctheatres.com), AMC spokeswoman Pam Blase said Tuesday. The ads, welcomed by the film's distributor, Universal Pictures, say in part: "AMC believes Cinderella Man is one of the finest motion pictures of the year!" Blase said AMC provides occasional rebates to...