Customer/Updater/CustomerUpdater.php
protected function updateCustomerBalance(PaymentInterface $payment, $amount = null)
    {
        if (null === $customer = $payment->getSale()->getCustomer()) {
            // TODO Deals with customer change
            return false;
        }

        $amount = $amount ?: $payment->getAmount();
        if ($this->isAcceptedPayment($payment)) {
            $amount = -$amount;
        }

        if ($payment->getMethod()->isCredit()) {
            return $this->updateCreditBalance($customer, $amount, true);
        } elseif ($payment->getMethod()->isOutstanding()) {
            return $this->updateOutstandingBalance($customer, $amount, true);
        }

        return false;
    }
protected function updateCustomerBalance(PaymentInterface $payment, $amount = null)
    {
        if (null === $customer = $payment->getSale()->getCustomer()) {
            // TODO Deals with customer change
            return false;
        }

        $amount = $amount ?: $payment->getAmount();
        if ($this->isAcceptedPayment($payment)) {
            $amount = -$amount;
        }

        if ($payment->getMethod()->isCredit()) {
            return $this->updateCreditBalance($customer, $amount, true);
        } elseif ($payment->getMethod()->isOutstanding()) {
            return $this->updateOutstandingBalance($customer, $amount, true);
        }

        return false;
    }
Updates the customer's balance regarding to the payment method.

@param PaymentInterface $payment
@param float            $amount The previous amount (update case)

@return bool Whether the customer has been changed.