I owe you one; I'm in your debt
Used to acknowledge a favor or help received, implying a sense of obligation to return it. Often used in casual conversation.
Thanks for helping me. I owe you one.
I'm in his debt, so next time it's my turn to help.
Means to return a favor or repay a debt, the action that follows when one has 借りができた.
Standard kanji-kana spelling for this phrase.
Alternate spelling using 出来る for できる.
From 借り (debt, obligation) + が + できた (came into existence), literally 'a debt has been created'. The phrase is a natural extension of the noun 借り used in the context of favors.