{By the mammon of unrighteousness} (ek tou mam(936e)(8320)t(8873)
adikias). By the use of what is so often evil (money). In  Mt
6:24  mammon is set over against God as in  Lu 16:13  below.
Jesus knows the evil power in money, but servants of God have to
use it for the kingdom of God. They should use it discreetly and
it is proper to make friends by the use of it. {When it shall
fail} (hotan eklip(8869)). Second aorist active subjunctive with
hotan, future time. The mammon is sure to fail. {That they may
receive you into the eternal tabernacles} (hina dex(936e)tai humas
eis tas ai(936e)ious sk(886e)as). This is the purpose of Christ in
giving the advice about their making friends by the use of money.
The purpose is that those who have been blessed and helped by the
money may give a welcome to their benefactors when they reach
heaven. There is no thought here of purchasing an entrance into
heaven by the use of money. That idea is wholly foreign to the
context. These friends will give a hearty welcome when one gives
him mammon here. The wise way to lay up treasure in heaven is to
use one's money for God here on earth. That will give a cash
account there of joyful welcome, not of purchased entrance.