(1) "No sweat."
Jack, can you lend me five dollars? Sure, no sweat.
(2) "It's not a big deal."
(3) "It's no big deal."
If you cannot come Friday night, it's no (not a) big deal.
Thank you very much! No sweat. It's no (not a) big deal.
(Note that in example #1, "It's no (not a) big deal" would not be used by a native speaker. In other words, the two expressions are not always interchangeable.)