I only just came across this conversation and i am running a little late. If i am repeating anyone else's solution, I appologize.
I have a certain group of contractors who i have worked with for years. a couple of them for over 30 years. i often give referrals but do not charge any commission for such. If i do refer one or two of them, i always call them to inform of the circumstances for the referral. I let them know, especially what i know of the person's pay habits.
If the contractor is going to be subbing under me, I tell them up front whether I will expect to charge my commission out of their charges or whether i am going to add my commission to their estimate and forward such charge to the customer. Either way, i tell my sub right up front which way i am going to do it. If i know the customer is going to be slow pay, i do not let my sub wait for his money and i add my commission to his charges. If i know a customer has a very bad reputation, i discuss with my sub about whether we really want the job.
Of course, I am slowly phasing back my biz so I am really not into work large enough to warrant sub contracting and any work i am accepting now is from long time tried and true customers. Within the last two weeks, I have actually "fired' the only customer who i have had any uneasiness about. From here on, any new sub work is for work i have been subbing to specific contractors for 100 years or so. All of my above explanation is not really appropriate for today, but that's how i have done in the past. The important thing for me is that i am upfront with my subs and we determine how any commissions are going to be arrived at before the sub presents his estimate to me.
Right now, i have to get on the "horn" to my very most reliable sub who has done my tree work since back in the '80's.
Tinker