Lets walk through some simple steps to know how can you achieve this !
Setting up Inventory Valuation Method
You can setup inventory valuation while adding new item from POS screen in iPad POS Register app. Head to accessories from POS screen --> Add new Item
Inventory AdjustmentTo adjust inventory quantity against any item with zero stock level, stock quantity can be adjusted when you tries to add that item to cart. It will give you an adjustment pop-up shown below where you can enter the adjustment quantity to make the transaction with that particular item.
Similarly, if you want to enter more than available stock quantity to cart, it will give you the same pop-up to adjust inventory quantity and you can upgrade your stock easily