Conclua a chamada AJAX antes que o botão do PayPal seja redirecionado

<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top" name="paypalform" id="paypalform">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="hosted_button_id" value="XXXXXXXXXXXX">
<button type="submit" class="radius paypal-button prevented" name="submit" id="paypalsubmit" alt="PayPal - The safer, easier way to pay online!">Buy Now!
<img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>

<script>
    $(document).ready(function(){
    $('#paypalsubmit').click(function(e) {
    if($(this).hasClass('prevented')){
        e.preventDefault();
        $(this).removeClass('prevented');
        $.post( "ajax.create_sponsorship.php",{ data: 'XXXXXX'}, function( result ) {
            if(result){
                $('#paypalsubmit').click();
                return true;
            }
        });  
    }else{
        $('#paypalform').submit();
        $(this).addClass('prevented');
    }
    });  
    })
</script>