disable cancel invoice in customer

lib/pages/invoice/payment/payment_page.dart

@@ -37,11 +37,13 @@ class _PaymentPageState extends State<PaymentPage> {
 
   bool isNew;
   File _file;
+  bool _hasBalance;
 
   @override
   void initState() {
     super.initState();
     _invoice = widget.invoice;
+    _hasBalance = widget.invoice.balance > 0;
     _loadInvoice();
   }
 
@@ -100,16 +102,18 @@ class _PaymentPageState extends State<PaymentPage> {
         body: ListView(
           padding: const EdgeInsets.all(10.0),
           children: <Widget>[
-            amountBox,
+            _hasBalance ? amountBox : Container(),
             SizedBox(height: 10),
-            Align(
-              child: Padding(
-                padding: const EdgeInsets.all(8.0),
-                child: receiptFileBox,
-              ),
-              alignment: Alignment.centerLeft,
-            ),
-            payBtnBox,
+            _hasBalance
+                ? Align(
+                    child: Padding(
+                      padding: const EdgeInsets.all(8.0),
+                      child: receiptFileBox,
+                    ),
+                    alignment: Alignment.centerLeft,
+                  )
+                : Container(),
+            _hasBalance ? payBtnBox : Container(),
             SizedBox(height: 15),
             LocalTitle(textKey: "pm.receipt"),
             Column(