App to redirect away from checkout if items are out of stock

[EllieCrawley]

I'm working with a store who have some very specific checkout requirements - they only sell products in multiples of 6. I can prevent customers from proceeding to checkout unless their basket contains the right number of items, but am having a problem with items become out of stock before they complete checkout, especially when they recover an abandoned cart some time later. This means that items are removed during checkout and they are now able to complete the purchase with the wrong number of items and the orders have to be manually adjusted later. Does anyone know of an app that can force a redirect from the checkout back to the cart page if items are removed due to being out of stock? Thanks.