One great thing about creating your own Change Price command is that you can control User Permissions on the button (Automation Command).
Automation Command:
As shown above in the Mapping for this Automation Command, only users belonging to the Admin or Manager role can see this button.
Action:
Action Type: Update Order
Price: [:Price]
Calculate Price: True
Rule: (create a default mapping for this rule)
Price Field: [?Price:\d+\.?\d{0,2};;ON]
In this Rule, the ? notation will cause SambaPOS to prompt the user for a value. The mask portion [\d+\.?\d{0,2}] is a regular expression that allows for only numbers to be entered, and limited to 2 decimals.