Genesis 15:2

And Abram said, Lord God, great blessings hast Thou given me, and great (are they which it is) before Thee to give me: nevertheless, what profit is to me, when I pass from the world without children, and Eliezer the manager (bar pharnasath, the son of sustenance) of my house, by whose hands signs were wrought for (or to) me in Darmasek, expects to be my heir?

AND ABRAM SAID, O LORD ETERNAL, WHAT WILT THOU GIVE ME? “Behold, Thou hast saved me from the kings, but Thou hast not assured me against extinction. Thou hast only said that Thou wilt give me great reward, but what can my reward be without children?”
Now it had not occurred to Abraham that this great reward would be in the World to Come for there is no necessity for such a promise; every servant of G-d will find life in the hereafter before him. But in this world there are righteous men, unto whom it happened according to the work of the wicked. It is for this reason that the righteous have need of assurance. Moreover, very great implies that he will merit both worlds with all the best therein without any punishment whatever as befits the really righteous people. Moreover, an assurance is given for that which a person fears. [Hence, he needed no assurance concerning the hereafter. But he feared being childless; therefore G-d] rejoined and explained that His assurance included that he should not fear this either, as He will make his children as the stars of heaven for multitude.
You may ask: Has it not been told to Abraham already, For all the land which thou seest, to thee will I give it, and to thy seed forever. And I will make thy seed as the dust of the earth, and so, how could Abraham now say, Since I go childless…lo, my household slave will be mine heir? And why did he not believe in the first prophecy, as he would believe in this [second one which G-d will now relate to him?] The answer is that the righteous ones have no trust in themselves, fearing they might have sinned in error. Thus it is written At one instant I may speak concerning a nation, and concerning a kingdom, to build and plant it; but if that nation turn and do evil before Me, then I repent of the good. Now when Abraham saw himself advanced in years and the first prophecy concerning him had not yet been fulfilled, he thought that his sins had withheld that good from him., 5:25. And perhaps he now feared that he would be punished for the people that he killed in the war, as our Rabbis have said. They have expressed a similar thought in Bereshith Rabbah:, 76:2. “Then Jacob was greatly afraid and was distressed. From this we derive the principle that there is no assurance for the righteous ones in this world, etc.”

?מה תתן לי, a reference to the reward in this life.

ערירי, “I have no son who can take my place.” Compare Jeremiah 22,30 כתבו את האיש הזה ערירי כי לא יצלח מזרעו איש יושב על כסא דוד, “record this man as without succession; one who shall never be found successful, for none of his offspring will be accepted to sit on the throne of David.” (the subject of this curse is כניהו son and heir of Yehoyakim)

ובן משק ביתי הוא דמשק אליעזר, he is only a slave, known only in his home town. There can be no question but that similar actions performed by a slave who is in constant fear of his master’s disapproval, even if in fact the same as those performed by the master’s son, are different in quality, and in effect from one another. The same chores performed by the son are motivated by love instead of by fear.

ויאמר אברם א-רני ה' מה תתן לי, the first attribute for G’d is spelled with the letter א-ד, an attribute, adjective, whereas the second name of G’d is the name used to describe the Essence of G’d, what we call Hashem for short. The adjective precedes the noun it describes in order to make clear that the adjective (G’d’s attribute) is in a construct mode of the noun (Essence) and not vice versa. It is not possible to convert the name of G’d the Essence into an adjective mode. If, nonetheless, we encounter such formulations on occasions, when it appears as if G’d’s name (the Essence) has been used as if it were an adjective, this merely reflects the way in which human beings use such terms, i.e. the fault is man’s not the Torah’s.

?מה תתן לי, Avram’s question did not relate to what G’d would give him as a reward in the hereafter, rather he wondered what meaningful reward there could be for him on earth seeing he had no children. Any reward in this life in the absence of children to leave it to, he considered meaningless. His reward would only be consumed by others, not related to him.

ערירי, “childless,” as rendered by Onkelos. If a member of my household will inherit me, even if it is my servant who is faithful to me, and a willing disciple, You have said to me “to you and to your descendants I will give it.”

ובן משק ביתי, he will become my heir, if I will not have a biological heir. He is, after all, completely at home in my house.

משק, the root of the word is שקק, is grammatically parallel to such words as מכס, whose root is כסס, the meaning of the word is מן השוק, i.e. someone who comes and goes, not someone whose roots are in my house. The root occurs in that sense in Proverbs 28,15 דוב שוקק, “a prowling bear.” Both the roots כסס and שקק describe similar activities.