The contract between growers and merchants for the exchange of grapes on the Champagne market is a long-term agreement based on quantities. Commitments on quantities are made for several years and negotiated individually between growers and merchants. Each year, prices are negotiated at the interprofessional level, the interprofessional committee including members of the growers union, members of the merchants union and a government commissioner. It turns out that industrial organization theory, and more precise mechanisms outlined by incomplete contract theory are relevant to the analysis of such contracts in which prices and quantities are negotiated sequentially and by different groups of actors. We show that imposed pricing by the interprofessional organization can, in some cases, balance the bargaining power between growers and merchants and thus increase social welfare. At a time when the European Common Organization of Wine Markets casts doubt on interprofessional organizations, this result tends to justify their positive role.