“The meaning is that, while this decaying old world lasts, it is not wrong to use worldly wealth, or even worldly tactics to make friends or good contacts which may lead to a more successful spreading of the gospel by virtue of us appearing as friendly and helpful to others (obviously, as long as serious Christian principles of integrity are not compromised), this is done in order that discipleship may be better served. Jesus is not talking about us all starting to play the financial stock market, taking huge risks, in order to hope for huge personal wealth. That would simply be a vain attempt to ‘store up treasure’ for ourselves upon earth. Rather, He is saying, use any tool you have (even mammon) to make disciples for Christ!
Interestingly, the NIV Study Bible suggests that the shrewd manager was quick to offer the creditors a better debt deal because he already knew that he had overcharged them in the first place! Hmmm, a good point.
The Amplified Bible makes the following of Luke 16:9:
And I tell you, makes friends for yourselves by means of unrighteous mammon (deceitful riches, money, possessions), so that when it fails, they (those you have favored) may receive and welcome you into the everlasting habitations (dwellings).
Of course nothing here excuses the money-motivated behaviour of the health, wealth and prosperity gospel merchants. They will surely come into judgment for using the gospel as merchandise in order to enjoy a comfortable living for themselves. Jesus was talking about something quite different. He was - effectively - saying, ‘Don’t be naive, learn to handle worldly/financial situations with a little acumen and common sense while this old world lasts, especially if it
might be advantageous to the Gospel. Eventually, this old world will be gone, then you will be welcomed into eternal dwellings, perhaps even welcomed by some you were able to help.’ He is certainly not saying that those who are a little
gullible and naive, too slow to play the world according to the world’s often devious tactics, will not be in those eternal dwellings.”
(Taken from http://www.ukapologetics.net/12/luke16.htm.)