I can see both sides to this..
On the one hand the agent may have experience of delayed payment due to funds not being released and uses this as a method to agree up front.. it may even avoid disputes if there have been multiple listings and agents. On the other, nobody really enjoys using the services of an estate agent, unless they were wildly off on their own valuation. Hence I imagine they suffer quite a bit from the "whore's dilemma" with clients less happy to pay after the service is delivered.
I assume a lot of firms get referrals from agents, I can imagine in a small town its essential for lawyer and agent to get along.
My suggestions are not legal advice