To pay back a debt or to repay someone for a favor; literally to return what is owed.
お世話になった人にはいつか借りを返したい。
I want to repay those who helped me someday.
借りたお金は計画的に借りを返す必要がある。
You need to repay borrowed money according to a plan.
奨学金は卒業後に借りを返すことになる。
Scholarships (loans) must be paid back after graduation.
Idiomatic: to settle a score or take revenge; to get back at someone for a wrong—context determines whether it means revenge or repaying a favor.
彼にされた仕打ちにはいつか借りを返すつもりだ。
I intend to get back at him someday for what he did.
チームに負けた借りを返すため、来年は全力を尽くす。
To settle the score from losing to that team, we'll give our all next year.
借りを返そうとするあまり、行き過ぎた行動に出てしまった。
In trying to get revenge, he went too far.