In this article we study the allocation of a budget to promote an opinion in a group of agents. We assume that their opinion dynamics are based on the well-known voter model. We are interested in finding the most efficient use of a budget over time in order to manipulate a social network. We address the problem using the theory of discounted Markov decision processes. Our contributions can be summarized as follows: (i) we introduce the discounted Markov decision process in our cases, (ii) we present the corresponding Bellman equations, and, (iii) we solve the Bellman equations via backward programming. This work is a step towards providing a solid formulation of the budget allocation in social networks.