Hi Conga Clinic Thanks for reaching out to us here on the Community.
When you create the supplier credit the account that you usually should use is the account that the refund relates to/whatever the original account that was used when purchasing the item, we appreciate that you state that this isn't related to an existing bill. You could create a refund expense account if you wish from within the chart of accounts: Accounting > chart of accounts > New > in the 'Account Type' field select Other Expense > In the 'Detail Type' field enter Other Expense > In the 'Name' field enter Refund or something similar > save and close. Please remember to link both the supplier credit and the deposit by creating a Cheque payment from New+ > enter the suppliers name > add the two transactions > save and close.
Please note that we're not accountants and any information that we provide should be qualified by a professional.