[A_Brandybuck]

Sauron isn't dumb. He had no choice. He tortured Gollum and even so he didn't get the knowledge about the location of the One Ring. He only knew the words "Baggins" and "Shire". But he knew the power of the Ring. He knew, that the only thing, that Gollum will do in liberty is to search the One Ring.

In this moment Gollum is Sauron's only connection to the Ring. To let Gollum free and escape from Mordor is the only decision Sauron could make in the case of Gollum. And when Gollum was caught and hold in Mirkwood, Sauron had to fear, that Gollum will lead the Wise to his Ring. So the only logical decision was to free Gollum, that he won't lead the Wise to the Ring. Not because he feared that the Wise could destroy the Ring, but they could use it against him (as mentioned above).

But the only fault was, that Sauron didn't thought about destroying the Ring. When the Wise have decided to use the Ring, Gollum wouldn't have had the chance to get the Ring.