SALES DELIVERY BY FIFO METHOD

Suppose Customer u2018C01u2019 give me sale order first,after that my second customer u2018C02u2019 give me sale order of same Item that u2018C01u2019 give first but my employee give material to second customer first, I want to block the this process ,I want sales process like FIFO

So how it can be happened

Plz reply me