add the slot updateDisplayUnit() to overviewpage, sendcoinsdialog, sendcoinsentry and connect it to displayUnitChanged() - this ensures all fields in the GUI, who use a display unit are imediately updated, when the user changes this setting in the optionsdialog / ensure used fields init with the current set display unit

src/qt/sendcoinsentry.cpp

@@ -68,6 +68,10 @@ void SendCoinsEntry::on_payTo_textChanged(const QString &address)
 void SendCoinsEntry::setModel(WalletModel *model)
 {
     this->model = model;
+
+    if(model && model->getOptionsModel())
+        connect(model->getOptionsModel(), SIGNAL(displayUnitChanged(int)), this, SLOT(updateDisplayUnit()));
+
     clear();
 }
 
@@ -82,10 +86,8 @@ void SendCoinsEntry::clear()
     ui->addAsLabel->clear();
     ui->payAmount->clear();
     ui->payTo->setFocus();
-    if(model && model->getOptionsModel())
-    {
-        ui->payAmount->setDisplayUnit(model->getOptionsModel()->getDisplayUnit());
-    }
+    // update the display unit, to not use the default ("BTC")
+    updateDisplayUnit();
 }
 
 void SendCoinsEntry::on_deleteButton_clicked()
@@ -160,3 +162,11 @@ void SendCoinsEntry::setFocus()
     ui->payTo->setFocus();
 }
 
+void SendCoinsEntry::updateDisplayUnit()
+{
+    if(model && model->getOptionsModel())
+    {
+        // Update payAmount with the current unit
+        ui->payAmount->setDisplayUnit(model->getOptionsModel()->getDisplayUnit());
+    }
+}