In some ways Modern Greek is remarkably like Ancient Greek, especially Koine Greek. I am coming to appreciate this more and more as I learn Ελληνικά. There are, though, profound differences which have cropped up over the millennia. One of these is that our friend the dative case ceased to exist at some point post-Koine period. Over time the genitive and accusative cases came to take all the functions of the dative case, and people just simply stopped using it. Just one of those interesting vicissitudes of language development. During the Koine period evidence of the eventual demise of the dative case pokes through in many places, especially in less literary writing. The dative case losing ground seems to be a logical explanation for an interesting citation of the 10th commandment in the work Apostolic Constitutions.
The Apostolic Constitutions has a complex history. For the sake of summary, we will just say it reached its present form sometime toward the end of the AD 200-400 time period and appears to have been compiled and arranged from parts written at different times. These days, it is generally believed to hail from Syria. Historically, it was associated with Clement of Rome, which probably explains the significance it had in various circles for some time.
The genitive for dative of possession
The piece of Greek that interests me at the moment can be found in the following citation:
«Οὐκ ἐπιθυμήσεις τὴν γυναῖκα τοῦ πλησίον σου οὐδὲ τὸν ἀγρὸν αὐτοῦ οὐδὲ τὸν παῖδα αὐτοῦ οὔτε τὴν παιδίσκην αὐτοῦ οὔτε τοῦ βοὸς αὐτοῦ οὔτε τοῦ ὑποζυγίου αὐτοῦ οὔτε ὅσα τοῦ πλησίον σου ἐστίν»
You shall not long after your neighbor’s wife, nor his field, nor his male servant, nor his female servant, nor his oxen, nor his donkey, nor whatever belongs to your neighbor1.1.6-10, Constitutiones Apostolorum, Constitutiones apostolorum “Les constitutions apostoliques, 3 vols.”, Ed. Metzger, M. Paris: Cerf, 1:1985; 2:1986; 3:1987; Sources chrétiennes 320, 329, 336
While reading through the passage, it struck me as odd that the genitive τοῦ πλησἴον σου appeared here, as I was kind of expecting a dative. So I looked up the text in Ex. 20.17 and Deut. 5.21, both of which have the phrase in question with the dative: ὅσα τῷ πλησίον σού ἐστιν. They use the so-called dative of possessor, which is used
“to complement ‘existential’ εἰμί and γίγνομαι (γίνομαι)…denoting possession, belonging, or interest”Cambridge Grammar of Classical Greek sec. 30.41
The text as it stands in Apostolic Constitutions seems to be a departure from the LXX in the direction of developing Greek usage of the time, moving away from the dative case. For reference, the modern Greek translation of Spyros Filos has the following Greek in both Ex. and Deut.: που είναι του πλησίον σου. The Modern Greek is strikingly like the phrase as it appears in Apostolic Constitutions.
The full quotation of the 10th commandment in Apostolic Constitutions does not match either the passage in Ex. 20 or Deut. 5 as they stand in Rahlf’s-Hanhart LXX. It is most like Ex. 20.17, but it begins like the passage in Deut. 5.21 and it is missing a phrase found in both those texts: οὔτε παντὸς κτήνους αὐτοῦ. I don’t have the Göttingen LXX texts to check the apparatus and see if there are any known manuscripts which match the text as it stands in Apostolic Constitutions. Short of doing a study on the patterns of Scripture citations in Apostolic Constitutions, I can quickly note that in the immediate context there is a citation from the Gospels which is a conflation of two similar passages from Matthew and Luke. An intuitive guess is that the author/compiler of Apostolic Constitutions is largely if not entirely citing biblical passages from memory.
On the intuitive assumption, based on little in-depth research but beyond the level of complete guesswork, it is interesting that our author would produce a version of the 10th commandment that substitutes the genitive case for the dative case. He would essentially be recasting the text from memory into a Greek idiom that was more normal for the time, as opposed to one that was beginning to take on an archaic or learned flavor. This would require more intense looking into both his citations and the broader usage of the dative case in the document as a whole to try to further verify. As it stands, it is an interesting case of a case-interchange that happens to pop up in an interesting place: a citation of the tenth commandment.
 The dative ceased to be used in literary texts by the 10th century, outside of a few stereotyped phrases (Flavia Pompeo, “Dative,” in Encyclopedia of Ancient Greek Language and Linguistics (Leiden: Brill, 2014), 414). In its place we find the accusative, the genitive, or the prepositional phrase εἰς + accusative.