code: 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;
      }
docs: 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.