From William Lane:
8 In the one case, mortal men(k) exact a tenth, but, in the other case, one to whom witness is borne that he lives.(l)
And in the footnotes:
k. In the anarthrous construction ἀποθνῄσκοντες ἄνθρωποι, which is virtually adjectival, the present tense of the ptcp is frequentative, i.e., it refers to an action that recurs from time to time. It, therefore, means “men who are to die, mortal men” (Moulton, Grammar, 1:114).
l. The expression μαρτυρούμενος ὅτι ζῇ is described by Moule as “a bold and rather unusual way” of declaring “it is attested that he is alive” (Idiom-Book, 104–5).
From the commentary on verse 8-10
8 The second contrast between Melchizedek and the Levitical priests is drawn in v 8. The basis of the comparison remains that both the Levitical priests and the priest of Salem received tithes. The emphasis, however, has shifted from the question of qualification (v 3a, 5–6a) to that of the relative duration of their respective ministries. The two halves of the verse are set in sharp contrast by the construction καὶ ὧδε μὲν . . . ἐκεῖ δέ . . . , “in the one case . . . but in the other . . .” In v 8a the participle ἀποθνῄσκοντες, “mortal,” is placed before the noun it modifies to characterize the Levitical priests who receive tithes as mortal men. Those who are appointed to priestly service by the law are subject to death and so have a series of successors. Although the term δεκάτας, “tithes,” and the appropriate form of λαμβάνειν, “to receive,” are not repeated in v 8b, they are implied. It is this fact which justifies the application of the extraordinary ascription ὅτι ζῇ, “that he lives,” to Melchizedek. As the clause stands, all the emphasis falls on the startling assertion that Melchizedek is “one to whom witness is borne that he is alive.” The term μαρτυρούμενος, “witness is borne,” almost certainly has reference to Scripture (cf. v 17; 10:5). In this context the declaration must refer back to v 3, which the writer considered to be exegetically established on the basis of Ps 110:4 and Gen 14:18–20. Scripture announces of Melchizedek only his living and the administration of a priesthood that is free from temporal limitation (cf. Schröger, Verfasser, 143; Cockerill, Melchizedek Christology, 74–78; Demarest, History of Interpretation, 136). The basis of Melchizedek’s superiority to the Levitical priests in this second contrast is the “eternity” of Melchizedek predicated in v 3b, which has in view the perpetuation of his priestly office. The importance of this aspect of the argument will become clear in vv 15–16, where it is applied to the messianic priest. So far as the record of Scripture is concerned, Melchizedek has no end of life and his unique priesthood has no successor. But what is true of Melchizedek in a limited and literary sense is true absolutely of the one who serves his people as high priest in the presence of God (F. F. Bruce, 141–42).
9–10 The climax of the argument is reached in v 9 and qualified in v 10. It specifies the implication of the first contrast between Melchizedek and the Levitical priests (vv 5–6a) by deducing the deeper significance of the fact that Abraham allotted a tithe to Melchizedek (Cockerill, Melchizedek Christology, 23–24, 78). The literary phrase ὡς ἔπος εἰπεῖν, “one might almost say,” was frequently used when a writer broke off the train of his thought and, not wishing to treat his theme more fully, would summarize as succinctly as possible what he had to say. Here it indicates the writer clearly recognized his statement that Levi had paid a tithe to Melchizedek was not literally true, because at the moment in primal history when Abraham met Melchizedek Levi was as yet unborn. Nevertheless, the statement that Levi had himself paid the tithe was true in an important sense, indicated by the expression διʼ Ἀβραάμ, “through Abraham,” which immediately follows. The corporate solidarity that bound Israel to the patriarch implied that Levi was fully represented in Abraham’s action. Therefore, Levi’s status relative to Melchizedek was affected by Abraham’s relationship to that personage. Consequently, the superiority of Melchizedek over the Levitical priesthood is not merely theoretical but has a basis in history (cf. Riggenbach, 190–91; Williamson, Philo, 107–9; Cockerill, Melchizedek Christology, 78–80). The assertion in v 9 is justified and explained in v 10, as shown by the explanatory conjunction γάρ, “because.” Although Levi was as yet unborn when Melchizedek met Abraham, the tithe Abraham gave to Melchizedek was a gesture that anticipated the subordination of Levi and the Levitical priesthood to the priesthood like Melchizedek’s that would be inaugurated at God’s appointed time.
Lane, William L.. Hebrews 1-8, Volume 47A (Word Biblical Commentary)