How to key in new purchase with the created item but not same quantity and item cost?

Let's divide your question into two: 1) how to set up the quantity of the purchased item? 2) how to change the price of the item during sale.

1) You can change the quantity of the items by tapping the selected item when it is on the receipt. Change the quantity in the popup window.

2) If you left the ‘Price’ field of the item blank, you could set the price during a sale.