Hi Josef,
I am assuming you are using traditional web app.
You can pass id of your button to the web block and when you do any change in web block (like onkeypress extended event of input) you can disable/ hide your button (with the help of button id you have passed) using javascript. Javascript code can be like below
<script>
document.getElementById('"+ButtonId+"').disabled = true;
</script>
In you event handler on screen after you assign the date to local variable use runjavascript server action from HTTPRequestHandler ext to enable the button using same javascript above with .disables = false
I haven't tried it myself but I think this should work