What are the connotations of the words "u'Tzedakah Tih'yeh lanu"?
Ramban: It is a Chesed on the part of Hashem that, in spite of the fact that we are obligated to perform His will, He nevertheless rewards us for doing so. 1
Targum Onkelos: Performing the Mitzvos also provides us with merits.
Ramban: Like a master who rewards his Eved for serving him even though the Eved is obligated to do so anyway. Mishnas R. Aharon (4, p.64)