A question that bothered me every time I'm watching FMA\FMAB, it is clear that one body & soul had to be paid (plus arm), but why Al's and not Ed's? anytime I asked myself that I told myself that it happened to be that way, but it could be the opposite and Ed's body & soul to be taken and there's no special reason to that.

And that is my point, is there a reason or it really happens to be that way for no reason at all (except the author reasons of course ...).