Struggling with weak sales, Volkswagen United States Inc. is offering a 30-day money back guarantee on its mid-size Passat automobiles. Anyone unsatisfied with a Passat car or station wagon in the first 30 days or 3,000 miles of ownership may return it and get their money back, Bill Young, vice president in charge of Volkswagen United States, said Wednesday. Other automakers have offered or are offering similar guarantees, although none allow a buyer to walk away with their money. Oldsmobile, for example, is offering to let buyers of its 1990 models who are not satisfied after the first 30 days or 1,500 miles of ownership return a car for credit toward the purchase of another Oldsmobile. At VW, Gelgota said, ``You get the full refund, sales tax, title and license fees and everything. It's your cash back.'' VW doesn't think it is risking its money, Gelgota said. ``It's based on our research. Ninety-seven percent of our customers are so satisfied they would recommend the Passat to a friend or relative.'' VW will purchase any returned cars from dealers, Gelgota said.