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You have to keep in mind that Gollum was of no value to Sauron as another slave in Mordor. So why not release him and see what happens? If he's not able to take the Ring again, he at least can be a bug in the fur of his foes and a hindrance to whatever they are planning. Which he was in the beginning and in the end.
Not only was it beyond Sauron's imagination that his enemies would try to destroy the Ring (did he overestimate his Ring, or underestimate his enemies?), but he also couldn't imagine that Gollum would actually end up helping them. This came by the pity of Frodo, which is well beyond the scope of Sauron's comprehension as well.
Given Sauron's knowledge and views at the time, Gollum's release makes perfect sense to me. In hindsight, things are always different.
By the way, do we know Sauron was behind the attack on Mirkwood? Did he even know Gollum was captured?
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