A few steps.

Step 1:  Fix the supplier account. 

1a.  If Payables is saying that the supplier owes you $, then create an 'invoice' for this same amount, Expense = Misc Expenses, Paid by = Pay Later.

1b.  If Payables is saying that you owe the supplier $, then create an 'invoice' for the same amount, but NEGATIVE, again Expense = Misc Expenses, Paid by = Pay Later.

1c.  'Explain' to Payables that the invoices cancel each other out by entering a dummy payment.  If you open the 'make a payment' window, you should see two invoices with balances outstanding that cancel each other out (say, $15 and <$15>).  Select both of them for payment, and double check that the total you are paying is zero.

Step 2:  Fix your Mastercard payable account and your Misc Expenses accounts.  Do this using a general journal entry.  Look at both of these accounts, and figure out what they should be.  Then, make a general journal entry to fix the problem.

Thank you for your reply. Unfortunately Sage does not bring up the Mastercard Payable account for me to debit to "fix" when posting a general journal entry like you suggested for step 2. I don't know why it won't show it for me, even when I click "show all accounts".

Thank you for your reply. Unfortunately Sage won't show my Mastercard Payable account to allow me to debit to "fix" it when creating a general journal entry. Even when I click "show all accounts". 

Is your Mastercard payable a general ledger account, or is Mastercard a supplier in your accounts payable list?