"... Positively, Matthew’s addition of the article is still significant. So why did Matthew add [an] article to the LXX? This can be explained in a number of ways: (1) textual corruption - Matthew used a Greek translation of the Old Testament or a Hebrew text that included the article, but is no longer extant; (2) a memory slip—Matthew alluded to Psalm 37:11 from memory and unintentionally added the article since in the LXX, κληρονομέω often takes the article + γῆ as its direct object; or (3) an intentional change - Matthew consciously, intentionally modified the Greek and/or Hebrew texts available to him (or memorized by him) for his own purposes. The best explanation is probably #3 along with the relevant biblical data from #2: The addition of the article allowed Matthew to bring his allusion to Psalm 37:11 into conformity with the overarching use of κληρονομέω + the article + γῆ in the rest of the Old Testament, thus making clear that Matthew 5:5 refers to inheriting the land of Israel. In other words, Matthew adds the article to build the specific combination [κληρονομέω + the article + γῆ], a combination that significantly narrows down the possible meaning of τὴν γῆν in Matthew 5:5 ..."

Hsieh, Nelson S. Matthew 5:5 and the Old Testament Land Promises: An Inheritance of the Earth, or the Land of Israel? (pp. 41-75) Master's Seminary Journal, Vol. 28, No. 1, 2017