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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
財布を忘れた同僚の昼食代を立替えてあげた。
I paid for my colleague's lunch since they forgot their wallet.
出張費用は一旦立替えて後で精算する仕組みだ。
Business trip expenses are paid upfront and reimbursed later.
友人のチケット代を立替えたがまだ返してもらえない。
I fronted the money for my friend's ticket, but still haven't got paid back.