Here is the passage in Luke 6 expanded for the sake of context:
Luke 6:27-28 27 “But I say to you who hear, love your enemies, do good to those who hate you, 28 bless those who curse you, pray for those who mistreat you. 29 Whoever hits you on the cheek, offer him the other also; and whoever takes away your coat, do not withhold your shirt from him either. 30 Give to everyone who asks of you, and whoever takes away what is yours, do not demand it back. 31 Treat others the same way you want them to treat you. 32 If you love those who love you, what credit is that to you? For even sinners love those who love them. 33 If you do good to those who do good to you, what credit is that to you? For even sinners do the same. 34 If you lend to those from whom you expect to receive, what credit is that to you? Even sinners lend to sinners in order to receive back the same amount. 35 But love your enemies, and do good, and lend, expecting nothing in return; and your reward will be great, and you will be sons of the Most High; for He Himself is kind to ungrateful and evil men. 36 Be merciful, just as your Father is merciful.
37 “Do not judge, and you will not be judged; and do not condemn, and you will not be condemned; pardon, and you will be pardoned. 38 Give, and it will be given to you. They will pour into your lap a good measure--pressed down, shaken together, and running over. For by your standard of measure it will be measured to you in return.”
Matthew Henry writes:
"If we are of a giving and a forgiving spirit, we shall ourselves reap the benefit of it: Forgive and you shall be forgiven. If we forgive the injuries done to us by others, others will forgive our inadvertencies. If we forgive others’ trespasses against us, God will forgive our trespasses against him. And he will be no less mindful of the liberal that devise liberal things (v. 38): Give, and it shall be given to you. God, in his providence, will recompense it to you; it is lent to him, and he is not unrighteous to forget it (Heb. 6:10), but he will pay it again. Men shall return it into your bosom; for God often makes use of men as instruments, not only of his avenging, but of his rewarding justice. If we in a right manner give to others when they need, God will incline the hearts of others to give to us when we need, and to give liberally, good measure pressed down and shaken together. They that sow plentifully shall reap plentifully. Whom God recompenses he recompenses abundantly.
III. We must expect to be dealt with ourselves as we deal with others: With the same measure that ye mete it shall be measured to you again. Those that deal hardly with others must acknowledge, as Adoni-bezek did (Jdg. 1:7), that God is righteous, if others deal hardly with them, and they may expect to be paid in their own coin; but they that deal kindly with others have reason to hope that, when they have occasion, God will raise them up friends who will deal kindly with them. Though Providence does not always go by this rule, because the full and exact retributions are reserved for another world, yet, ordinarily, it observes a proportion sufficient to deter us from all acts of rigour and to encourage us in all acts of beneficence."
Henry, M. (1994). Matthew Henry’s commentary on the whole Bible: complete and unabridged in one volume (p. 1844). Peabody: Hendrickson.
Henry appeals to God's providence as the final arbiter. He writes, "Though Providence does not always go by this rule, because the full and exact retributions are reserved for another world..." Scripture also tells us in Matthew 6:19-21, "19 “Do not store up for yourselves treasures on earth, where moth and rust destroy, and where thieves break in and steal. 20 But store up for yourselves treasures in heaven, where neither moth nor rust destroys, and where thieves do not break in or steal; 21 for where your treasure is, there your heart will be also." Ultimately, our treasures are spiritual, not material. But yes, God can choose to bless us materially in this life. But does He always? How do we reconcile Luke 6:38 when a believer who exercises true and obedient faith is not blessed materially? This is why Matthew Henry made an appeal to God's providence and this is what the prosperity hucksters get wrong. There is no quid pro quo; no act of faith on our part that obligates God to bless us materially.