I believe your question is based on your broker contacting the appraiser directly. There are new regulations that are in effect to protect the consumer. When the market turned and everyone was looking at who to blame for the escalation of price the appraisers took a lot of blame who then blamed others maybe like your broker who then tried to coerce them into bringing the value in. Do we hire an attorney? And tell them what exactly?
From your question, it's unclear exactly what happened. Or, more specifically, why the buyer's bank won't finance the sale. I'm guessing it's because your home didn't appraise for the contract price.
If your home didn't appraise for the contract price, ask your Realtor for advice. Depending on the contract, it's possible that the buyers have an "out" in that they can't get financing. You could always reduce the price of your home to the amount of the appraisal, if you're willing and able to do so. Or you could lower it some, and the buyers (if they're able) might put more money down.
But ask your Realtor for guidance.
If the seller's agent was unduly trying to influence the appraiser's findings, that would have been unethical. However, if it was a contact made in order to facilitate an appraiser being done, it should not have been a problem.