this will take a little hand coding to do.
1) Add a hidden form element to the add to cart form to store the available quantity, name it "QuantityOnHand"
2) Make note of the quantity form elements name. For sake of example, I will use "WA_Store_Cart_1_Quantity_Add"
3) Add the code at the top of the page:
<?php
if (isset($_POST['WA_Store_Cart_1_Quantity_Add']) && isset($_POST['QuantityOnHand'])) {
if($_POST['WA_Store_Cart_1_Quantity_Add'] > $_POST['QuantityOnHand']) {
header("location: ". $_SERVER['PHP_SELF']."?qty=toMany");
}
}
?>
This will redirect the page back to itself if they enter to big a value in the Quantity.
you can use this code to show an error message if you like:
<?php if(isset($_GET['qty']) { ?>
Your error message here
<?php } ?>