Aroo!
Have a card selected, but not looked at. Place the card on top of the deck, then remove it and show it. Place the card back on the deck, and turn slightly away from the first spectator and show the card...
Now, as you turn back, push the card on the top of the deck slightly off (still holding chosen card in other hand. Now, turn back to the first spectator and at the same time, place the selected card on top of the deck removing the jogged card.

Give the card, face down, to the spectator. While doing this, you can finish any way you wish. A suggestion is to palm the selected card, have the spectator return the other card (whish they think is the selected card) and have the deck shuffled. Then, while grabbing the deck, place the palmed card on top. If you have good hands, you can replace the palmed card into the deck face up. Simply fan the deck and close it casually slipping the chosen card in.