My understanding, when you dispose of an asset you do 2 things
Write off the residual book value allocating the cost to the “Loss on Disposal” profit and loss account (an internal business decision)
If the item has a residual real value, sell it and allocate the proceedings of the sale to the same “Loss on Disposal” account (a transaction with an external party for a price independent of any book assets current book value)
After which the asset has zero value on the Balance sheet accounts. And the Profit and loss accounts show the net cost or profit from disposing of the asset.
I’m not sure it makes sense to apply profit / loss from disposal & sale of an asset to a balance sheet account as that would leave a residual book value for an asset you no longer have.
The reason the fixed asset balance sheet account can be selected from receipt / payment is that is how the purchase is recorded when the business first bought the asset.
Thinking about it I suppose it comes done to the order step 1 and step 2 are done in / supported.
An alternative is to support selling an asset (allocating profit to the asset account, then disposing the asset (allocating any profit or loss to a “Loss on Disposal” Profit & loss account).
Manager appear to currently support order initially described but maybe there is a case to support the opposite order.
I’m not sure about the accuracy of the Balance sheet when a sales invoice and book value apply concurrently. It maybe confusing to users as if sold above book value that would results in a negative book value when disposed in Manager.