Try this one and check just changed type:'POST' in ajax request.
<script type="application/javascript">
$(document).ready(function () {
$('#btnUsingjQuery').click(function () {
$.ajax({
type: 'POST',
url: "http://paypal.com/payment/widget?apikey=XXXXX&country=my&returnurl=http://www.google.com",
dataType: 'jsonp',
success: function (results) {
window.location = 'http://paypal.com/payment/widget?apikey=XXXX&country=gb&returnurl=http://www.google.com';
},
error: function () {
window.location.replace("http://www.google.com");
}
});
});
});
</script>