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to pay for someone else (with the expectation of being reimbursed later); to lend someone the money to pay for (e.g. rent); to make a temporary payment on someone's behalf
財布を忘れた友人の分を立て替えた。
I paid for my friend who forgot their wallet.
出張費用を立て替えて後で精算する。
I will pay the business trip expenses upfront and settle later.
立て替えた金額は来月返してください。
Please return the amount I paid for you next month.