Fieldmist Borderpost

• WWizards of the Coast2009-05-01
  Official Ruling
  Effects that increase or reduce the cost to cast this card will apply to whichever cost you chose to pay.
• WWizards of the Coast2009-05-01
  Official Ruling
  Casting this card by paying its alternative cost doesn't change when you can cast it. You can cast it only at the normal time you could cast an artifact spell. It also doesn't change the spell's mana cost or mana value. The only difference is the cost you actually pay.
• WWizards of the Coast2009-05-01
  Official Ruling
  To satisfy the alternative cost, you may return any basic land you control to its owner's hand, regardless of that land's subtype or whether it's tapped.
• WWizards of the Coast2009-05-01
  Official Ruling
  As you cast a spell, you get a chance to activate mana abilities before you pay that spell's costs. Therefore, you may tap a basic land for mana, then both spend that mana and return that land to your hand to pay this card's alternative cost. (Of course, you can return a different basic land instead.)

Rulings via Scryfall