Data mining techniques managing imprecision are very useful to obtain meaningful and interesting information for the user. Among some other techniques, fuzzy association rules have been developed as a powerful tool for dealing with imprecision in databases and offering a good representation of found knowledge. In this paper we introduce a formal model for managing the imprecision in fuzzy transactional databases using the restriction level representation theory, a recent representation of imprecision that extends that of fuzzy sets. This theory introduces some new operators, keeping the usual crisp properties even when negation is involved. The model allows us to mine fuzzy association rules in a straightforward way, extending the accuracy measures from the crisp case. In addition, we introduce several ways of representing and summarizing the obtained results, in order to offer new and very interesting semantics. As an application, we present how to extract fuzzy association rules involving both the presence and the absence of items using the proposed model, and we also perform some experiments with real fuzzy transactional datasets.