Previous works on vote-buying have highlighted that an informational advantage allows party machines to efficiently distribute discretionary transfers to voters. However, the microfoundations that allow party machines to electorally exploit their informational advantage have not yet been elucidated. The probabilistic model in this paper provides the microfounded mechanism that explains how party machines translate, with a voter-customized strategy, their informational advantage into more efficient allocation of discretionary transfers and win elections with higher probabilities than their contenders. Furthermore, its probabilistic design allows the model to account for why party machines target their own supporters with discretionary transfers. In-depth interviews with 120 brokers from Argentina motivate the model.