Promise of Payment

Promise of Payment
The next card you play this turn costs ⓪. Pay its gold cost during your next upkeep or lose the game.
PurpleFuture Magic

Card-Specific Rulings
You will get gold from your workers before you have to pay the cost. Also, since you as the active player decide in what order your upkeep effects happen, you can collect gold from upkeep effects like Galina Glimmer and Gemscout Owl before you have to pay. HOWEVER, you can't play cards or activate abilities (such as Merfolk Prospector and Rickety Mine) until your Main phase, so they can't help you pay the cost. — Sirlin
You still have to pay any additional costs of that card as you play it. For example, if your opponent has a Building Inspector, you pay 1 even if Promise of Payment makes the first building you build cost 0. — Sirlin
Hiring a worker is not playing a card. Building a Tech Building or Add-on isn't playing a card. Using an ability of a card or putting a card into play is not playing a card. Promise of Payment doesn't apply to any of those things. — Sirlin
You CAN use Promise of Payment to play a unit from Graveyard or to play the top card of your draw pile with Vir's middle ability. — Sirlin
Be sure to play Promise of Payment JUST before you play the card you want to pay 0 for. If you play another card in between (even with an effect like Cinderblast Dragon's attack effect), you have to apply Promise of Payment's effect to that card instead. — Sirlin
The gold cost you have to pay during your next upkeep is the printed gold cost on the card. You don't get to apply any effects that would reduce its cost (even Gigadon's effect that's on the card itself). — Sirlin
If for some reason you play Promise of Payment and then don't play any other cards that turn, nothing happens during your next upkeeep. — Sirlin
Promise of Payment does not help you pay for Boost. If you play Promise of Payment and then Murkwood Allies, you only get one kind of token unless you pay 4 gold. — sharpo