Amount field shows insufficient funds error based on amoutn + gas total.

ui/app/conversion-util.js

@@ -132,12 +132,16 @@ const conversionUtil = (value, {
 });
 
 const addCurrencies = (a, b, options = {}) => {
-  const { toNumericBase, numberOfDecimals } = options
-  const value = (new BigNumber(a)).add(b);
+  const {
+    aBase,
+    bBase,
+    ...conversionOptions,
+  } = options
+  const value = (new BigNumber(a, aBase)).add(b, bBase);
+
   return converter({
     value,
-    toNumericBase,
-    numberOfDecimals,
+    ...conversionOptions,
   })
 }
 

ui/app/send-v2.js

@@ -12,7 +12,11 @@ const GasFeeDisplay = require('./components/send/gas-fee-display-v2')
 
 const { showModal } = require('./actions')
 
-const { multiplyCurrencies, conversionGreaterThan } = require('./conversion-util')
+const {
+  multiplyCurrencies,
+  conversionGreaterThan,
+  addCurrencies,
+} = require('./conversion-util')
 const { isValidAddress } = require('./util')
 
 module.exports = SendTransactionScreen
@@ -225,12 +229,19 @@ SendTransactionScreen.prototype.validateAmount = function (value) {
     conversionRate,
     primaryCurrency,
     toCurrency,
-    selectedToken
+    selectedToken,
+    gasTotal,
   } = this.props
   const amount = value
 
   let amountError = null
 
+  const totalAmount = addCurrencies(amount, gasTotal, {
+    aBase: 16,
+    bBase: 16,
+    toNumericBase: 'hex',
+  })
+
   const sufficientBalance = conversionGreaterThan(
     {
       value: balance,
@@ -239,7 +250,7 @@ SendTransactionScreen.prototype.validateAmount = function (value) {
       conversionRate,
     },
     {
-      value: amount,
+      value: totalAmount,
       fromNumericBase: 'hex',
       conversionRate: amountConversionRate,
       fromCurrency: selectedToken || primaryCurrency,