Instead of doing this with calculation, I would use a custom parsley validator. Although calculations could be set up to prevent the issue like Matthew mentioned, I think a validator has a lot more flexibility.

For example, if you use a validator, you could make it so the form wouldn't submit even if the button was visible because it would fail validation unless the password fields matched.

In the code below, I have a "passwordInput" class set on the main input, and a "passwordConfirm" class set on the read-only field. I also included code to hide/disable the button in comments.

$(document).ready(function(){
  //$('#myButton').hide().prop('disabled',true);
  
  // assign validator to password field
  $('.passwordInput input').attr({
    'placeholder':'Enter your password',
    'type':'password',
    'data-parsley-passwordconfirmation':''
  });
});

// custom validator for password confirmation
window.Parsley.addValidator('passwordconfirmation', {
  validateString: function(value, requirement, field) {
    var password = $('.passwordInput input').val();
    var confirm = $('.passwordConfirm input').val();
    
    var correct = (password == confirm && confirm != '');
    //$('#myButton').toggle(correct).prop('disabled',!correct);
    
    return correct;
  },
  messages: {
    en: 'Incorrect password.',
  }
});

Now that being said, doing a password this way is not especially secure, especially since someone could inspect the page and find the comparison field.

However, if security isn't a concern and you're using a lookup to pull the password based on the username, then I think a better way would be with a stored procedure.

Instead of populating the form with a value someone could find, build a stored procedure in your database that accepts username and password as the parameters.

Then, have that procedure return a true/false or 0/1 value to say whether or not a match was found and pull that value into your form instead.

That way, you can have field rules to show/hide the button based on whether or not the stored procedure returns a good value and you don't have the password somewhere they can check.