The traditional theory of exchange implicitly assumes instantaneous exchange at the contractually determined price. However, actual exchange may entail payment before or after supply as well as determination of effective price ex ante or ex post of supply. The paper demonstrates that the market determination of price timing arises in conjunction with the resolution of mutual uncertainty among buyers and sellers concerning product quality and product cost. The conditions are identified under which ex post pricing will result rather than the more traditional ex ante pricing. The theory accounts for the simultaneous use of ex post and ex ante pricing in a single market. Legal service pricing is used to illustrate the theoretical principles derived in the paper.