The verse instructs us to “Bless the Lord your G-d for the good land” (Devarim 8:10). Our Sages (Berachot 48b) extrapolate from here that the Torah requires three blessings to be included in Birkat HaMazon. The first blesses G-d for providing food (Birkat HaZan), the second blesses G-d for the Land of Israel (Birkat HaAretz), and the third blesses G-d for the rebuilding of Jerusalem (Boneh Yerushalayim). If they are based on the verse, how can the Talmud also tell us that Moshe instituted the first blessing, Yehoshua the second, and David and Shlomo the third? It must be that while the content was established at Sinai, the precise words that we recite were formulated by Yehoshua, David, and Shlomo.
Since Birkat HaZan was instituted by Moshe Rabbeinu, it is surprising that some versions of the blessing include the verse: “You open your hand and satisfy the desires of every living thing” (Tehillim 145:16). After all, Tehillim was written by King David, who lived much later than Moshe. The likely explanation is that some of the verses of Tehillim were formulated at an early stage, and King David wrote them down at a later stage. This is borne out by the language used in Birkat HaZan to introduce the verse: “As it is said, ‘You open your hand,’” and not “As it is written, ‘You open your hand.’”
As we mentioned, the specific formulation of the blessings was originally different from what we recite today. A person could have fulfilled his obligation (for Birkat HaZan) by saying in Aramaic, “Blessed is the merciful One, King, the Creator of this bread.” Along the same lines, when someone sings Tzur MiShelo – the Shabbat song whose structure is parallel to that of Birkat HaMazon and incorporates the same themes – it is possible that he has fulfilled his obligation to recite Birkat HaMazon. Accordingly, perhaps a person should have in mind when he sings Tzur MiShelo that he does not intend to fulfill his obligation. This way, he ensures that his fulfillment of the mitzva takes place only when he recites the classic Birkat HaMazon.