Chris,
Here is how to complaint to the DOT
Complaints about airline service other than safety or security issues may be registered with DOT's Aviation Consumer Protection Division (ACPD). You can call, write or use the web form.
Call the ACPD 24 hours a day at 202-366-2220 Calls are returned Monday through Friday, generally between 7:30 am and 5:00 pm Eastern time.
Write to:
Aviation Consumer Protection Division, C-75
U.S. Department of Transportation
1200 New Jersey Ave, S.E.
Washington, D.C. 20590
You can send a complaint, comment or inquiry electronically by using the web form.
http://airconsumer.dot.gov/escomplaint/es.cfm
Whether you call, write or use the web form, be brief and concise in the description of your problem and be sure to include the following information:
your name
address
daytime phone number (including area code) and e-mail address
name of the airline or company about which you are complaining
flight date
flight number if known
origin and destination cities of your trip.
If you send a letter, you should also include a copy (not the original) of your airline ticket or itinerary sheet and any correspondence you have already exchanged with the company.
You should also consider asking Delta for a copy of the telephone call. Many calls are recorded and you may have the right to a copy. If you can obtain this, and can prove you were mislead, I think you may have a reasonable chance of winning in small claims, if you have such a thing in Argentina. You may also ask the DOT if they are able to obtain a copy.
Delta will defend this vigorously though. If airlines had to stand behind the lies their staff tell customers they would go bankrupt. (Oh I forgot, Delta did go bankrupt, but somehow in the US, that's no deterrent!!)