Musar zu Jehoschua 6:13

This is also why Moses in his blessing of the tribe of Gad in Deut. 33,21 refers to that tribe as having performed צדקת ה' עשה ומשפטיו עם ישראל, "both a deed of righteousness vis-a-vis G–d and one of justice did he (the tribe) perform with Israel." From a legal point of view the tribe of Gad did only have to take part in the conquest of the land, something which took seven years. The additional seven years that were required to parcel out the land was an act of צדקה, something volunteered. By honoring this voluntary undertaking, the tribe qualified for Moses' unstinting praise. The Torah wishes us to apply such modes of behavior not only when it concerns utterances that may be financially costly for us, but also when such utterances imply undertakings by our souls, i.e. our religious conduct.