This page explains how to start and complete the The Statue's Bargain side quest in The Legend of Zelda: Breath of the Wild.
How to Start the Quest
Near the entrance to Hateno Village, there is a flag on a pole on a hill. Go around that hill until you find a horned statue. Talk to it to start the quest.
How to Complete the Quest
After you start the quest, talk to the statue and say "give it back". Ask for a heart or stamina (the statue always takes a heart when you start the quest).
Quest Reward
From now on, you can talk to the horned statue to sell and buy heart containers and stamina. You still need to find Spirit Orbs to get more hearts and stamina, but now you can change hearts into stamina and stamina into hearts for a fee.