With Paymash version 1.2.29 you have access to the partial cancellation feature. Using this function you can cancel individual items from a bill instead of the entire bill as before.
Select the menu item "Bills" on the POS.
Then go to paid invoices.
Select the invoice for which you want to make a cancellation.
Go to "Cancellation" and select the products you want to cancel by tapping on the blue box on the right.