More than $600 within 90 days of filing for bankruptcy is a preference payment. File the petition and demand it back before filing an Adversary Proceeding. I recommend having an attorney do it, because the threat usually avoids the lawsuit. Either way, you need to file the petition first.
[This communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]
Please consult with an attorney.
The ability to recover the garnishment must be listed as property on Schedule B and exempted on Schedule C. Technically, the Trustee must abandon the property or you must wait until 31 days after the meeting of creditors because that is the deadline for the Trustee to object to the exemption. At that time, you can make a demand of the garnishee to return the funds, as long as they exceeded $600 in the 90 days prior to the filing of the bankruptcy petition. If they do not return the funds, you can file an adversary proceeding.
This answer is for information only and should not be construed as legal advice or as establishing an attorney-client relationship.
You need to file bankruptcy, have the ability to exempt the wages (which you would in DC), and then file an adversary proceeding if the creditor refuses to voluntarily return the money. To get started, you need to contact a DC-licensed bankruptcy lawyer asap.