In the first instance no allowance shall be made in the computation of the gain—
in the case of a disposal by way of assigning a lease of land or other property, for any liability remaining with, or assumed by, the person making the disposal by way of assigning the lease which is contingent on a default in respect of liabilities thereby or subsequently assumed by the assignee under the terms and conditions of the lease,
for any contingent liability of the person making the disposal in respect of any covenant for quiet enjoyment or other obligation assumed as vendor of land, or of any estate or interest in land, or as a lessor,
for any contingent liability in respect of a warranty or representation made on a disposal by way of sale or lease of any property other than land.
If any such contingent liability subsequently becomes enforceable and is being or has been enforced, there shall be made, on a claim being made to that effect, such adjustment, whether by way of discharge or repayment of tax or otherwise, as is required in consequence.
Subsection (2) above also applies where the disposal in question was before the commencement of this section.