How to say "I will make it up to you" in Japanese?
I can't seem to find a correct translation for this online. I would appreciate any help.

Question

Answer

(私はあなたに watashi wa anatani)

償います tsugunai masu.
埋め合わせをします umeawase o shimasu.
お返しをします okaeshi o shimasu.

...etc. They are used differently in situations. You can omit the first phrase (watashi wa anatani). A naked verb will do in Japanese.

I think
借りは返すよ kari wa kaesu yo.
is most suitable between male-male relations.

Answer

Then you can say
償いをさせてほしいんです
Tsugunai o sasete hosiin desu.

...Not to confuse you, I don't say many other words. Just say above phrase sincerely. She will never fail to accept your heart. Good luck !