You have the opportunity to set a default sales plan adjustment pricing rule for your rates in case you need to have a stable amendment. 


Simply go to Sales -> Sales plans and either enter an already existing rate plan or create a new one. On the settings tab you can then decide if you want to adjust the sales plan either by a fixed amount (e.g. +5€ or -10€) or with a percentage (e.g.. +/- 10%).