This is currently logged as an enhancement (Enhancement 232835: Fixed Assets > Transactions > Asset Proposal > Ability to Propose a Transaction that have a Credit or Negative Amount to an Asset Account).
As an alternative, the user may choose to process an asset revaluation:
For example, Asset Original Cost is 1,000.00 while Asset Current Cost is 900.00.
1. Navigate to Fixed Assets > Transactions > Asset Revaluation.
2. On the Asset ID/Name, enter the asset ID.
3. On the Write-Down Amount, enter 100.00.
4. On the Transaction Date, enter the date user wants to post the journal entry for the asset adjustment.
Once the asset revaluation is processed, user needs to create a manual journal entry to offset the G/L impact created by the asset revaluation.