Hi Natalie:

If I understand you correctly you paid your supplier with your finance/credit card company. Have you set up your finance/credit card company as a credit card in Sage? If so, then the payment will be credited to the credit card company. When you make a payment to the finance/credit card company your payment will be debited to the finance/credit card company and credited to your bank. Initially, you would have to enter the supplier invoice into payables and charge it to the proper expense/capital item and taxes. The interest can be added to payables and charged to the interest account on a monthly basis or however you receive the amounts of interest due.

Another way is make a journal entry charged to the proper accounts and credited to a long term loan account (you may have to create  a new ledger account). When you make your payments to the finance/credit card company, setup a vendor and charge to interest and principal/loan accounts and pay with your bank account or a credit card.

I hope this helps! Please follow-up if you have further questions!