Hello, AJAX its not calculating my if conditions - php

Question

Im making a AJAX form but i have a problem with validation, i need to make a condition when a input data is greater than a variable i define in the action file echo something out, but my condition is not working its always returning true, here is the code:
<?php
include "../../admin/includes/controller.php";
$msg = "";
$balance = "";
#Get stuff from form...
if(!empty($_POST)) {
#Define submited variables
$user = $_POST['user'];
$type = $_POST['type'];
$asset = $_POST['asset'];
$price = $_POST['price'];
$amount = $_POST['amount'];
$margin = $_POST['margin'];
#Check balance...
$sql = "SELECT * FROM users WHERE username='$user';";
$result = $db->prepare($sql);
$result->execute();
while ($row = $result->fetch()) {
$balance = $row['balance'];
}
This is the condition i want to check:
if ($balance > $margin) {
$msg = "Trade opened successfully";
} else {
$msg = "low balance";
}
#Print out message...
echo $msg;}
?>
This is my AJAX code:
$(document).ready(function () {
$('#open').click(function (e) {
e.preventDefault();
var asset = $('#asset').val();
var amount = $('#amount').val();
var margin = $('#margin').val();
var user = $('#user').val();
var type = $('#type').val();
var price = $('#price').val();
$.ajax({
type: "POST",
url: "q/db.php",
data: {
"asset": asset,
"amount": amount,
"price": price,
"type": type,
"user": user,
"margin": margin
},
success: function (data) {
$('.result').html(data);
$('#contactform')[0].reset();
}
});
});
});