Mills, 23, was recently cleared by the Chinese Basketball Association to return to the NBA. The contract is expected to be done within the next 24 hours after the Portland Trail Blazers renounce Mills' rights.
Mills played 74 games for the Trail Blazers from 2009-11, making 35.8 percent of his 3-point attempts. He is a native of Australia. Spurs assistant coach Brett Brown has coached Australia's national team.
Mills had also considered signing with the Rockets.
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