Legal Question in Real Estate Law in California

The issue should not come up because the attorney is required to give the client all the files, whether or not the legal fee billing has been paid. The attorney, you, are in trouble if you fail to do so. You have to tell the client that; you will have more headaches in dealing with the State Bar, which often can be a kangaroo court, then in not collecting the rest of the fee. People who feel their doctor or attorney has acted rudely to them are more likely to file malpractice suits.

Whether the client put signed under duress on the receipt is irrelevant; those circumstances are not an example of duress. Whether or not those or similar words are put on the receipt does not effect the ability to file a malpractice suit.

The attorney is not permitted to hold the file until payment is made. Client files and property must be returned to the client after the attorney client relationship ends, regardless of whether there is an outstanding bill or not.

I doubt an attorney would sign something that says payment was made under duress, as this implies threats, which are improper.

I agree. The files must be returned, payment or no. Further, "duress" has a technical meaning that doesn't apply here; there is no "duress" without violence or the threat thereof. If you have a fee dispute, you can demand arbitration - ask the county bar association for the county where the legal work was being done. If material errors were made by the attorney and you suffered losses as a direct consequence, you may also have an action for malpractice.

Previous answers seem to assume you are the attorney. It sounds to me like you are the client. If you think you have malpractice claims against the attorney, pay nothing and demand your file. Report the attorney to the State Bar complaints office as well. It is a significant ethical violation to demand payment before delivering the file.