Sale scheme

hi, we had sold a stock of 200 pack of juice to the distributor, and now stock comes to expires in August, so we decided that we run a scheme of one buy one get one free, in this case, the distributor pay us only for 100 pack and question is,
sale invoice generate for 200 pack, and we need to adjust 100 pack, how we show 100 pack in your accounts book, sale return or what.