Is there a way to add reward points?

I would like to add give reward points for each item purchased by customer. These rewards will be convertible to cash.
Say Customer A purchased 2 X Product 1 and 1 Product2.
Product1 has 10 points and product2 hs 5 points.

Can i add this feild to the customer invoice and track it?

One way to do this would be to issue credit notes (Credit Notes tab) to customers who have earned rewards. The amount from credit note will be automatically applied to their next invoice.