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This paper analyzes the properties of the English auction with bidding credits used for E-procurement in the case of differentiated goods. Assuming that production costs are increasing with the level of quality and that the score function of the buyer is concave, we characterize the optimal procurement mechanism when the level of quality offered is common knowledge. We show that the English auction with bidding credits implements the optimal mechanism only under specific conditions. When the private information on the costs is uniformly distributed, we characterize a modified English auction with bidding credits by capping payments and we prove that this procedure implements the optimal mechanism.