expression
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.
手伝ってくれてありがとう。借りができたね。
Thanks for helping me. I owe you one.
彼には借りができたから、今度は私が助ける番だ。
I'm in his debt, so next time it's my turn to help.
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.