Common practise in my expeinece - its probably a matter of cashflow. If your employers pay you before they receive the cash from the customer they are out of pocket. It may not seem much to you but when you consider other employee's commissions it may add up to a fair bit. I have worked both ways 1) you get paid the commission when you sell and 2) you get paid the commission when the company gets the money. 1) is preferable as you get it sooner and you can always know how much you are earning each month. But it comes down to cashflow as I said.
The alternative in your situation is that they pay you your commission but they will be entitled to claw it back from you if the customer doesn't pay your eomployers. I should imagine your company are not trying to avoid paying you but trying to avoid paying you if there is a good chance they will have to take it back (and believe me this is not good as its long spent by the time this happens). Speak to the credit controller in charge and see what the situation with this client is - you never know you may be able to help resolve the situation and get paid.