What is the ruling on the surplus amounts that a collector finds after inventorying and matching receipts, given that he has a "change" float and that accounting errors are deducted from his salary?

These sums of money belong to their rightful owners from whom they were mistakenly taken. They must be returned to them if they are known. Otherwise, they take the ruling of a found item (luqatah). If the amount is small and its owner would not pursue it, then there is no harm in taking it and benefiting from it. But if it is an amount that its owner would pursue, then it must be announced. If the owner does not come forward, then there is no harm in benefiting from it, and it must be returned to him if he appears. The deduction made by the shop owner from your salary does not justify taking this money.