'Can I get your opinion please?
3 καὶ τὸ οὖν Ἰσραὴλ ὄνομα τοῦτο σημαίνει· ἄνθρωπος νικῶν δύναμιν· τὸ γὰρ ἴσρα ἄνθρωπος νικῶν ἐστι, τὸ δὲ ἢλ δύναμις. ὅπερ καὶ διὰ τοῦ μυστηρίου τῆς πάλης, ἣν ἐπάλαισεν Ἰακὼβ μετὰ τοῦ φαινομένου μὲν ἐκ τοῦ τῇ τοῦ πατρὸς βουλῇ ὑπηρετεῖν, θεοῦ δὲ ἐκ τοῦ εἶναι τέκνον πρωτότοκον τῶν ὅλων κτισμάτων, ἐπεπροφήτευτο οὕτως καὶ ἄνθρωπος γενόμενος ὁ Χριστὸς ποιήσειν.
Chapter 125: "...That Christ ... in that Christ ministered to the will of God, yet He is God, because He is the First-begotten of all creatures..." (KET)
CHAPTER CXXV:[III]: "...And that Christ ... in that He ministered to the will of the Father, yet nevertheless is God, in that He is the first-begotten of all creatures..." (R&DT)
θεοῦ δὲ ἐκ τοῦ εἶναι τέκνον πρωτότοκον τῶν ὅλων κτισμάτων
"(of) God but out-of (of) the to be a child first-born (of) the (of) entire/whole (of) created things"
What is your opinion?'
If I understand your question correctly, you are asking about how the bold part of the quote from Justin's Greek text has been translated. Frankly, I can see that the Greek is not rendered literally, but I don't personally believe that it has been mistranslated. At least, not the part in bold. δὲ is functioning adversatively here. It can be rendered "but," "however," or "yet" in this context.
The translations you quote above have also left out τέκνον in their renderings, but that choice does not pose a major difficulty for me. I notice that one translation has "He ministered to the will of the Father" where the other says "Christ ministered to the will of God." These renditions may be due to textual variants, although the sense of the passage remains fairly the same. Let me know if I have understood you correctly.