The best business relationships can become friendships - but, by definition, they exist initially for each party to gain some business advantage.  There is probably a spectrum of sorts in the strength and meaning of relationships (e.g. from acquaintances to alliances to friends to partners); and friendship is a natural extension of relationships that work because of personal rapport.  But, when the order is reversed and you are looking to do business with existing friends, then it pays to formalise the arrangement. Put in place a written letter of engagement at a minimum, defining objectives, approach, budget and timing and the payment process. Be clear on scope and any assumptions that will be material to your success.  All reasonable, all standard practice, no surprises. Formalising business relationships won't ensure no problems but it will minimise the risk that problems occur due to misunderstanding or that one party comes to feel exploited by the other.