in is my view that your schema should differ so that the Asset table is more generic, and includes the Add/Scrap records i.e.:
tblAsset
AssetKey - autonumber
AssetID - does your organization assign IDs to assets?
AssetName
ActionDate
ActionValue
ActionType - Acquisition, Add, Scrap
Then you can group by AssetID (or by AssetName if truly always unique) and the ability to sum a total at any date is very simple.
You can present/report Acquisitions vs Adds vs Scraps easily & separately by report/query criteria based on the Action Type. ... such that users would not know, nor need to know, that the data is held in a common table.
this is a fairly standard approach to inventory management, and in general it is better to calculate & present a sum rather than calculate & store a sum, as in terms of best practices one must then be very careful that a stored sum does not become out of sync with changing data....