How is it that the shekel is considered HEKDESH immediately upon giving it. If this is so, how is that change can be given back from that money, wouldn't the change also be considered HEKDESH?
The Mishnah later (6a) states that if someone proclaims that he will bring his Shekel from "this" money, he may keep the amount of money that exceeds the value of a Shekel. The reason is that he only intended to give the value of a Shekel from that money and not more. Similarly, in the case where a person gives a large coin to the treasury of Hekdesh, since he only intended to give the value of a Shekel, only that value becomes Hekdesh. The change he receives (that is, the amount of money that exceeds the value of a Shekel) is therefore not Hekdesh.