jQuery: Set event.preventDefault within GET or POST Request

I have a submit form on my page which only should be submitted if a specific condition is met.

The problem is that this condition is not yet fixed at the time of executing the script. Therefore, I want to reload it with a simple jQuery .get() or .post(). Depending on what the result of this reload is, a event.preventDefault should be set (for preventing the submit) or not.

Up to now, I have elaborated the following JavaScript code:

$("#formid").submit(function(event){
    // request
    $.post("script.php",$(this).serialize(),function(msg){
        // prevent submit in case of specific content
        if (msg=='xx') event.preventDefault();
    });
});

However, there is a problem with that code: it is just another working and I have no idea why! If I set the prevenDefault outside the POST (or even GET request, I have tried both), it is working like a charm. But I need it within the POST or GET request, because there is the source of my condition.

Can someone help me and give me an approach for solving the problem?