The sin was accounted by Yehovah to Aaron. Deuteronomy 9:20 "And Yehovah was very angry with Aaron to have destroyed him: and I (Moses) prayed for Aaron also the same time. 21 And I took your (the People) sin (according to Moses), the calf which you had made, and burnt it with fire, and stamped it, and ground it very small, even until it was small as dust: and I cast the dust thereof into the brook that descended out of the mount." Was this a transfer of the accounting of this sin? Transferred from Aaron to the People, from one man to the nation that forgiveness may come, yes grace? We know that the sins of the nation of Israel, indeed of the whole world were transferred to Yeshua HaMashiyah, He who "takes away the sins of the world." Sin was transferred to one man from the nation (and the world) to "declare His righteousness for the remission of sins that are past..." (Romans 3:25). This becomes a parable, a reflection, a mirror image of the judgement and righteousness of God, transferring sin from one man (Aaron) to the nation, and reversed from the nation to one man (Yeshua).The Brook that "descended from the mount" is the water of life, which Yeshua gives which ye "shall never thirst" Jn 4:14. Moses therefore ground up the gold to dust, to transfer the sin to the "Brook", which is a similitude of Yehovah. Jer 2:13 "For My People have committed two evils; they have forsaken Me the fountain of living waters..." (flowing waters, not a cistern); Jer 17:13 "O Yehovah, the Hope of Israel, all that forsake Thee shall be ashamed... because they have forsaken Yehovah, the Fountain of living waters". The gold of Egypt is "in the Brook", that is where the Holy knowledge is, even the hope of eternal life that the Pharoahs believed in, where the sons of Israel had been enslaved. The answer therefore is to transfer the sin to the Brook, an act of faith by Moses to declare Yehovah's ultimate plan for His People through The Son of God, to remind Yehovah of His plan that Moses understood well by now, and I would say because it may have been revealed to Moses "in the mount". Many times Moses skilfully avoided the awesome judgement of Yehovah upon these People as is written in Deut 9:22 on. The tribe of Levi nevertheless executed judgement for Yehovah upon 3000 people with sword fights between the men (Ex 32:26-35) After this event, Yehovah summoned Moses back up the mount to receive the two tablets of stone the second time and then separated the sons of Aaron from amongst Israel to bear the Ark of the Covenant and to stand before Yehovah to minister to Him, and to bless in His name (Deut 10:8), so Moses was successful in obtaining grace for Aaron and his sons.