In order to ensure that your desired margins are met that you have established in the setup of the Pricebook, the option for technicians using the Tech App to alter the prices is solely allowed by applying a promo code by default.
You can create Promo Codes in your Admin Portal through the Promo Code report in the Reports Center.
Should you need to manually adjust the prices of tasks without applying a promo code, an Admin Portal user with Admin or Super permissions will have to perform the price override -OR- you can enable a setting in Company --> Settings which will allow technicians to override the prices in the Tech App.
Adjustments that your staff makes to the prices established in the pricebook are tracked in the Sold Line Items report in the Price Adjustment column. Details about the pricing adjustments are found in the notes of the job where the adjustment was made.