Accumulated depreciation is cumulative depreciation value of an asset which is appearing on fixed asset register i.e. asset is not retired from fixed asset register (removed).
So every fixed asset schedule you will find Gross Block i.e. cost acquisition, depreciation for the year, accumulated depreciation and net block. In other net block (the current value of fixed assets) is gross block minus accumulated depreciation.
Depreciation rates are calculated as per income tax guidelines, they have different rates than companies act. So financials prepared and what you see in annual report is adjusted for tax calculation.
When asset is retired i.e. either sold or made obsolete, accumulated depreciation and asset cost is reversed from fixed asset register. Difference between sale value and cost of asset minus accumulated depreciation is considered as profit or loss may be the case.
Example: I purchases a machinery for 1 lac on 01.04.15. Depreciation for 2015-16 was 20000, for 2016-17 was 15000. Here accumulated depreciation is 20000+15000=35000. The net block is is 65000 i.e. 1 lac minus 35000.
Now imagine asset is sold for 75000. First and foremost 75000 recorded as cash into bank being received. 10000 difference between sale value 75000 and net block 65000 considered as profit on sale of asset and recorded as profit. 1 lac removed from fixed asset register, 35000 accumulated depreciation as well. Depreciation and net block automatically becomes zero.
For income tax: depreciation is charged off as expenses as per income tax rates. The profit on sale is considered as income and put to tax.
Interest on income tax arises from delayed payment of tax, nothing do with fixed asset sale specifically.
Lenders if provided the loan for acquiring fixed asset will be taken care as per lending agreement. If the agreement says no disposal without clearance one has to follow. It depends on clauses mentioned there.