Fieldmist Borderpost

You may pay {1} and return a basic land you control to its owner's hand rather than pay this spell's mana cost.
This artifact enters tapped.
{T}: Add {W} or {U}.

Archenemy

You may pay {1} and return a basic land you control to its owner's hand rather than pay Fieldmist Borderpost's mana cost.
Fieldmist Borderpost enters the battlefield tapped.
{T}: Add {W} or {U} to your mana pool.

Alara Reborn

You may pay {1} and return a basic land you control to its owner's hand rather than pay Fieldmist Borderpost's mana cost.
Fieldmist Borderpost comes into play tapped.
{T}: Add {W} or {U} to your mana pool.

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.)

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.

Effects that increase or reduce the cost to cast this card will apply to whichever cost you chose to pay.

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.