Smart money refers to money that is used and invested in wise ways.
In fact, the wise or smart use of money is one measure of a growing Christian's spiritual maturity. Many Christians are good stewards of their time and talent but not of their treasure. First of all, it is important to notice that in verse 8 it was the master in the story , not the Lord Jesus Himself, who praised the unrighteous manager. Furthermore, the unrighteous manager was not praised for his shady accounting procedures or squandering his boss's money. He was commended only for his prudent foresight in looking out for his own future.