He says they should use it to gain friends for themselves, so that when it is gone they will be welcomed into heaven (16.9). In this case the friends will not give them a job as the manager may have hoped. In this case the friends will welcome them into heaven. The reason they will do this is because the disciples will use their wealth to help others get into heaven. They will share their wealth so that others can hear the gospel and come to know Christ. These are the friends who will welcome them.
This is a test to see if they can be trusted with money. If they can show that they will not waste it (like the younger son or the bad manager) they can be trusted with true riches (16.11).
The main idea is in 16.13: is money and wealth what you trust and serve and depend on, or do you serve God and use the wealth he has given you for his work?
The Pharisees were not impressed (16.14) because they loved money. What about you?
Dale