You can keep track of any discounts that have been given to customers by running the Promotions and discounts report. 

Go to the reports tab and select the Promotions and discounts report.

Select the date range you require.

This will provide a list of all the discounts made in this date range and shows the value of these discounts. 

You can export and/or print this report by using the buttons on the top left of the page.