Free Goods determination in Invoice

Hi Experts,

My query is regarding Free Goods. Free Goods are determined in the sales order. Now lets assume that I have maintained free goods scheme for product A as 10 qty free on 100 Qty.

At the time of delivery, if insufficient stock (60 Qty - batch B) is there, my delivery qty will get reduced to like:

Main Item = 50 Qty

Free Item = 10 Qty

Kindly note that batches are automatically determined in the delivery and only one batch B with qty 60 is available.

Now at the time of creating invoice, I wish to check the scheme that the main qty and free goods qty should be in exact ratio as maintained in the condition master for free goods.

My question is how to restrict invoice creation in case of mismatch in main item qty and free item qty.

Thanks.