There are two sets of records of departing passengers on Ancestry.com. I do not think this is a comprehensive list of passengers, since those records were not required by INS.
U.S., Atlantic Ports Arriving and Departing Passenger and Crew Lists, 1820-1959 | Ancestry® [This list is for smaller ports only]
Dear Ms. Zimmer,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located the Population Schedules for the 1930 Census, and the Population Schedules for the 1940 Census in the Records of the Bureau of the Census (Record Group 29) that may contain information about the individual in Pennsylvania. The 1940 Census schedules are digitized and available using the Catalog. For more information about the non-digitized schedules, please contact the National Archives at Washington, DC - Textual Reference (RDT1) via email at firstname.lastname@example.org.
You may wish to search Ancestry or FamilySearch for the U.S. Census. There may be a fee for using Ancestry. Instead, please check for access at your local library as many library systems subscribe to these sites, making them free for their patrons.
Since he lived in Allentown, PA, he may have left from New York City or Philadelphia. We also located the Registers of Vessels Departing from New York, New York, , 8/11/1917 - 12/31/1971 (A3487) in the Records of the Immigration and Naturalization Service (Record Group 85) that may include the ship he departed on. For more information, please contact RDT1.
Next, we located the Manifests of Outward Bound Alien Passengers, 1907 - 1948 in Record Group 85 that consists of registers of immigrants departing from the port of Philadelphia. For more information about these records, please contact the National Archives at Philadelphia (RE-PA) at email@example.com.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT1 and RE-PA. We apologize for this inconvenience and appreciate your understanding and patience.
We suggest that you review NARA’s Resources for Genealogists, as well as the History Hub Blog titled Suggestions and Advice for Family History Researchers. Also, the FamilySearch Research wikis for Germany Emigration and Immigration and Pennsylvania Genealogy may be useful.
For more information about his return to Germany, we suggest contacting the Bundesarchiv, Postfach 450 569, 12175 Berlin, Germany.
We hope this is helpful. Best of luck with your research!
[Some information provided by Elizabeth Burnes, Subject Matter Expert]