Is this what you are trying to do Disposing of a part of a fixed asset - worked example
For Manager to dispose of an asset, the asset (as you have defined it) must be disposed. The solutions linked describes how to split an asset at disposal time into the item which has actually been disposed and the residual which is retained.
By the way, what you have defined as Fixed assets, I would define as control accounts. Then define fixed assets for each physical item, grouping then by the new control accounts. Totals can be displayed in the summary page as describe in the detailed link in my first post above.