Checked the standard coding and just copy how standard do the validation.

/*
         * <summary>
         * Check if the customer has exceeded credit limit
         * </summary>
         */
        internal bool hasExceededCreditLimit(Sage.Accounting.SalesLedger.Customer customer)
        {
            bool isovercreditlimit = false;
            decimal zero = 0;
            if(customer.TradingAccount.CreditLimit > zero)
            {
                if ((customer.PendingNotificationDocumentsValue + customer.AccountBalance) <= customer.TradingAccount.CreditLimit)
                {
                    isovercreditlimit = false;
                }
                else
                {
                    isovercreditlimit = true;
                }
            }
            return isovercreditlimit;
        }

Checked the standard coding and just copy how standard do the validation.

/*
         * <summary>
         * Check if the customer has exceeded credit limit
         * </summary>
         */
        internal bool hasExceededCreditLimit(Sage.Accounting.SalesLedger.Customer customer)
        {
            bool isovercreditlimit = false;
            decimal zero = 0;
            if(customer.TradingAccount.CreditLimit > zero)
            {
                if ((customer.PendingNotificationDocumentsValue + customer.AccountBalance) <= customer.TradingAccount.CreditLimit)
                {
                    isovercreditlimit = false;
                }
                else
                {
                    isovercreditlimit = true;
                }
            }
            return isovercreditlimit;
        }