Your first textbox doesn't matter if you're disabling it. So do something like-
$("#secondtxt").keyup(function(){
var val = $(this).val(), result = 0;
if(val < 5000){
result = 5000 - val;
}
else if(val > 5000){
result = "";
$(this).val('');
}
$("#resultTxt").val(result);
});
Pretty simple, eh ?
-KR