free while in stock - but pay to have it backordered

I would like to offer a free product while the product it is in stock. Once the stock is 0, I would like to then charge a price per product when it is on backorder.

For example the first 50 items in stock are free and any additional products ordered after 50 will be invoiced.

Is this possible as an automatic fuction or should I simply allow them to order the entire stock, and then manually replentish with a purchase option? (i.e. no longer free item)?

help would be great on what alternate ways I can accomplish this delima.

You could customize the code that when the products_quantity is > 0 then the Product is marked as Product is Free YES ...

Then, when the stock hits <= 0 it could turn off the Product is Free ...

Then, when you get more instock you update the quantity and mark the product as Product is Free YES ...

I'm wanting my customers to take all that is on the shelf for free. If I only have 5 on the shelf, I'd like them to have to pay for any products over the 5 I have in stock. So, the only time they would have to pay is if they ordered more than what I have on the shelf. The ones on the shelf would be free and the invoice would contain any additional "back ordered" products as a additional per item charge.

I guess the only way to do this is to make them order 2 times. One which empties the shelf and gets shipped, then the second order for any "back order/special order" amounts I turn on after the initial stock is emptied? I guess another way would be to have the item listed twice. Once with a 20 qty for free and a second listing showing a higher quantity of stock that would be charged. Max out the 20 frees and allow backorder on the pay per item?

What about running a "free" sale on a set amount like 20 and any additional items after those sale items would be billed. That way if I have 50 on my shelf, run a sale on 20 of them and the remaining 30 no longer have a sale.

Possible? Confused?

You could write software to achieve this effect, but there's nothing built in that would do this.