Jamieson, Fausset and Brown on Psalms 106:31

31. counted ... righteousness--"a just and rewardable action."

for--or, "unto," to the procuring of righteousness, as in Ro 4:2; 10:4. Here it was a particular act, not faith, nor its object Christ; and what was procured was not justifying righteousness, or what was to be rewarded with eternal life; for no one act of man's can be taken for complete obedience. But it was that which God approved and rewarded with a perpetual priesthood to him and his descendants (Nu 25:13; 1Ch 6:4, &c.).