Via custom JavaScript verification:
$(document).ready(function(){
var total = 100;
var sum = $('#ConstantSumQ_div .question_body tbody tbody > tr:last-child input').val();
var remaining = total - sum;
if (remaining > 0)
{
strErrorMessage = 'You need ' + remaining + ' more.';
}
})
You will need to set the custom JavaScript verification to run before the system JavaScript verification, rather than after.