Ok - I think I get it.
I am not sure what service you provided but likely you are the TPA that administers the group contract for this company and the insurer either lost the contract to another insurer or exited the group voluntarily (bad experience, not a strategic business, etc). As an insurer, we record the expense paid to a TPA at the point its incurred which is when the premiums are actually collected.
It seems to me that the "receivable" you have is a contingency payment that is triggered when the premiums are paid in. My reaction is you cant set up a receivable until the premiums are paid so I would not estimate the receivable but instead accrue it as you earn that fee.
As an insurer, we recognize premium when its due and not assume future collectibility. On single pay contracts we defer the premium and recognize over the benefit period. In all cases we are not permitted to recognize a receivable for future premium until its actually due. I would apply the same rule for you as you are not due any payment day 1 but instead are earning it over a period time that depends on the premiums collected.
Take a look at SAB 104 and see if this helps. Its says "revenue should not be recognized until it is realized or realizable and earned; and revenues are considered to have been earned when the entity has substantially accomplished what it must do to be entitled to the benefits represented by the revenues". SAB 104 lays out the conditions to meet this criteria and also has some fairly good examples. On page 74 it describes a medial arrangement where they collect a fixed fee and are not permitted to recognize the revenue before the underlying collection has occurred even though it can be reliably estimated - this is similar to your case and I say you need to wait for the premium to be paid before you recognize your service fee. Nothing as specific as yours but similar in the facts. I dont believe you have completed the revenue process to entitle you to recognize revenue day 1. I cant upload the file but here is the link
http://www.sec.gov/interps/account/sab104rev.pdf
If I have the facts wrong PM me and we can take this offline.