I believe the party you are referring to happens in the opening scenes, as Oskar Schindler is trying to make friends with the SS in Poland so he can use them as business contacts.  So he buys all the good stuff on the black market and pays for this lavish party with wine and food at a nice restaurant to get himself started.  He also stages a party for  Amon Goeth, the Pleszow Labor Camp Commandant, who's help he needs to continue to operate his business.  He buys him women and wine and stages a party at his villa in the camp to curry favor and "reward" Goeth for his help.  In both cases, though, it is Schindler himself who pays for the parties.

I feel that a bit more detail might be needed in such a situation.  There were many parties in the early phases of the work, and specificity might help out in providing a full answer to the question.  That being said, my initial guess would be that Schindler himself is paying for the party.  At the start of the work, Schindler believes that spending lavish amounts of money in order to procure favors from those in the Nazi hierarchy of power is critically important to him becoming wealthy.  Some of this involved hosting parties where food, drink, and revelry was evident as guests who attended the party were more likely to give Schindler "preferential treatment," in its most positive sense, as opposed to what the term would come to mean for Jewish individuals and those deemed as enemies of the Nazis at the time.