If you put away money for shikalim saying, ‘these should be for my shikalim’, and you didn’t realize there was a surplus, Bais Shamai says the surplus becomes a present for the Bais Hamikdash, Bais Hillel says it can be taken back.

However, in the same case by the chatas everyone agrees the leftover is a present to the Bais Hamikdash.

Shikalim 2:4

R’ Shimon says the difference between chatas and shikalim is that shikalim has a specific amount where as chatas does not. R’ Yehoshua says even shikalim doesn’t have a specific amount as they always used a ½ of the coin used at that time (not necessarily a shekel). R’ Shimon says no, because it does have a specific amount of whatever coin is used that year.