You would need to loop through the repeater for each entry until you find the value that matches the label. Or you could do it all at once
$value_lookup = array();
while (have_rows('repeater')) {
the_row();
$value_lookup[get_sub_field('label')] = get_sub_field('value');
}
then you can use this array to look up the values
$value = $value_lookup[$label];