MAGETWO-91769: Credit Memo - Wrong tax calculation! #10982

- Converting base order shipping amount for correct calculation;

Author: Alexey Yakimovich

app/code/Magento/Sales/Model/Order/Creditmemo/Total/Tax.php

@@ -75,10 +75,11 @@ public function collect(\Magento\Sales\Model\Order\Creditmemo $creditmemo)
         }
 
         $isPartialShippingRefunded = false;
+        $baseOrderShippingAmount = (float)$order->getBaseShippingAmount();
         if ($invoice = $creditmemo->getInvoice()) {
             //recalculate tax amounts in case if refund shipping value was changed
-            if ($order->getBaseShippingAmount() && $creditmemo->getBaseShippingAmount() !== null) {
-                $taxFactor = $creditmemo->getBaseShippingAmount() / $order->getBaseShippingAmount();
+            if ($baseOrderShippingAmount && $creditmemo->getBaseShippingAmount() !== null) {
+                $taxFactor = $creditmemo->getBaseShippingAmount() / $baseOrderShippingAmount;
                 $shippingTaxAmount = $invoice->getShippingTaxAmount() * $taxFactor;
                 $baseShippingTaxAmount = $invoice->getBaseShippingTaxAmount() * $taxFactor;
                 $totalDiscountTaxCompensation += $invoice->getShippingDiscountTaxCompensationAmount() * $taxFactor;
@@ -104,7 +105,6 @@ public function collect(\Magento\Sales\Model\Order\Creditmemo $creditmemo)
             }
         } else {
             $orderShippingAmount = $order->getShippingAmount();
-            $baseOrderShippingAmount = $order->getBaseShippingAmount();
 
             $baseOrderShippingRefundedAmount = $order->getBaseShippingRefunded();