if you purchase a r/t on delta and not use the return they can charge you for the one-way price of the trip you took if it is higher.
Rule 100G
… a ticket is invalid: … if Delta Air Lines determines that the ticket has been purchased or used in a manner designed to circumvent applicable fare rules. Where a ticket is invalidated as the result of the passenger’s non-compliance with any term or condition of sale, Delta Air Lines has the right in its sole discretion to: Cancel any remaining portion of the passenger’s itinerary, Confiscate unused flight coupons, Refuse to board the passenger or check the passenger’s baggage, or Assess the passenger for the reasonable remaining value of the ticket, which shall be no less than the difference between the fare actually paid and the lowest fare applicable to the passenger’s actual itinerary.
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f