SQL Server 2008 :: Pulling Daily Total From Cumulative Total

I have a table that writes daily sales each night but it adds the day's sales to the cumulative total for the month. I need to pull the difference of todays cumulative total less yesterdays. So when my total for today is 30,000 and yesterday's is 28,800, my sales for today would be 1,200. I want to write this to a new field but I just can't seen to get the net sales for the day. Here is some sample data. For daily sales for 6-24 I want to see 2,000, for 6-25 3,000, 6-26 3,500, and 6-27 3,500. I'm thinking a case when but can't seem to get it right.

CREATE TABLE sales
(date_created date,
sales decimal (19,2))
INSERT INTO sales (date_created, sales)
VALUES ('6-23-15', '20000.00'),
('6-24-15', '22000.00'),
('6-25-15', '25000.00'),
('6-26-15', '28500.00'),
('6-27-15', '32000.00')