The sale of an asset for tax purposes is something that you learn at F6
Assets are subject to ‘special’ tax rules called capital allowances and, where a sale of an asset gives rise to a profit … that profit is ignored for tax purposes
Instead the proceeds of sale are deducted from the tax written down value of that asset and the difference (if proceeds are less than wdv) gives rise to a balancing allowance
If proceeds exceed wdv, then a balancing charge arises