Money paid temporarily on behalf of someone else (a cash advance or payment made in advance), usually to be reimbursed later.
出張費は会社が立替金として一度支払った。
The company paid the travel expenses once as an advance.
立替金は領収書を提出すれば後で清算されます。
Advance payments will be settled later if you submit the receipts.
友人の分を立て替えたので、立替金を返してもらった。
I paid for my friend's share in advance, so I got reimbursed later.