Delta Air Lines - Fare Decrease After Ticket Purchase
Domestic General Rules Tariff DGR-1
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It happens every so often. You purchase an airline ticket only to find later, that the airline has lowered the fare, announced a sale, or instituted a new fare. When this happens, you can receive a refund - even on non-refundable tickets.

If, after you have purchased your Delta Air Lines ticket and before you begin your trip, there is a decrease in the fare or a lower fare is offered, you may receive a refund of the difference provided:

You do not change the origin, destination, stopover point, flights or dates shown on your ticket.
You meet all the requirements of the new fare (advance purchase, minimum stay etc).
If you purchased a fare that imposes a fee for changes, this fee will generally be waived if you accept your refund in the form of a non-refundable Delta Air Lines travel voucher. The voucher is good for one year from the date of issuance and can be used towards the purchase of future air travel or air travel related services on Delta Air Lines.

Example: You purchased a $500 non-refundable ticket that has a $75 change fee. Before you begin your trip, a $350 fare is offered. If you make no changes to your trip (origin, destination, stopover points, flights and dates) and you meet the requirements of the new fare (minimum stay and advance purchase), you may have your ticket re-written and receive a refund in one of two ways.

If you want your money back, you will receive a refund for $75 ($150 for the difference in the fare, less $75 change fee).
If you accept your refund in the form of a Delta Air Lines travel voucher, the $75 change fee is waived and the travel voucher is issued for $150.
There may be times when Delta Air Lines offers special fares that are available for a limited period of time to which this refund rule does not apply. i.e. special internet or sale fares.