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