Why Sometimes debtor's A/c Showing Credit Balances ?

debtors paid advance showing debtor account credit balance

debtors paid excess amount  debtor account showing credit balance

now a days goods delivery after they are make payment sometimes excess amount paid debtors then next piont is creditor make a credit note so, debtor ledger shows credit balance against our scheme for a expamle cash discount & slaes target like wise ..

When Debtors paid Advance Against the Contract / purchases then the debtor shows a credit balance in the books.

When the client pay in advance , or if the client pay before invoicing . 

Normally the receivable account shows debit balance but when the amount is received in advance then it shows a credit balance