In my opinion the difficulties in relations with friends in the business questions arise from the fact that these issues are resolved amicably. I.e. with business partners in business relations there are no problems with friends in a friendly, don't you.
The fact is that when friends enter into a business relationship, they both hope for a friendly relationship. For example, the party that needs to fulfill some commitments calculates that he will pay more (for example) and that if there is a slight delay in timing, anything terrible will not be — "because we're friends." But at the same time, the other side is counting on the opposite — that the obligations were fulfilled in time and that would have to pay could be less, "because we're friends."
According to this, I think the most important thing in such matters to say it. To talk and discuss all the nuances. Go out with business partners, we negotiate all the details to chance? Why do friends do this?
When I lend money to an unfamiliar person, I do not forget to specify all the nuances and why would he the money and when he will give them to me, and when borrows a friend, to ask such questions as would be ugly — of course then it turns out that someone is something wrong!
T. O. that would not lose friends in the business, all the questions you need to discuss out loud and not to rely on telepathy from the other, and to draw his attention that you are not a mind reader. I.e., in cases in which friends need to communicate in business :).