The wording of the feat Deadly Dealer is as follows:
You can throw a card as though it were a dart, with the same damage, range, and other features. You must use the Arcane Strike feat when throwing a card in this way, or else the card lacks the magical force and precision to deal lethal damage. A card is destroyed when thrown in this way.
Harrow cards are treated as masterwork weapons when thrown using this feat, but are still destroyed after they are thrown. A harrow deck can no longer be used as a fortune-telling device after even a single card is thrown.
A spellcaster with this feat can enhance a deck of cards as though it were a ranged weapon with 54 pieces of ammunition. This enhancement functions only when used in tandem with this feat, and has no affect on any other way the cards might be used.
Only a character who possesses this feat can use an enhanced deck of cards; she must still use the Arcane Strike feat to activate the cards' enhancement. (Emphasis Mine)
The Card Caster Magus archetype's modified Arcane Focus Pool states:
A card caster's arcane pool can be used to augment only ranged weapons. At 5th level, a card caster can use his arcane pool to add the following weapon special abilities to ranged weapons: brilliant energy, distance, flaming, flaming burst, frost, icy burst, returning, seeking, shock, shocking burst, and speed. He can use his arcane pool to augment an entire harrow deck as if it were ammunition. All cards from a deck enhanced with a special ability, such as flaming, must share the same bonus. This ability modifies arcane pool. (Emphasis Mine)
Obviously, the ability allows you to augment ranged weapons other than Harrow Cards, but what happens when you use returning on a Harrow Card? Does the property make it so it no longer breaks when destroyed, as long as the card is imbued with the returning property, or does the "specific trumps general" nature of "Cards are destroyed when thrown" trumps the general of "thrown objects return when enhanced"?