A client may have a global discount assign so when you are making a sale it will be apply to the whole process. Now you should do the following on the device.
Main menu Dashboard > Settings
Sales
Discount Structure > Type
Select Specific Client (%)
Now you need to apply it on a client
Main Menu > Database > Clients
Select the client
Touch Discount - type the percentage then Save
Remark: When making a new sell, all the items on the order will apply the discount given to the client based on the default price A