You need a tax adviser to answer this one but logically you could argue that so long as the retention was a true cost at the time and could not have been included as profit, it should not have been taxed but carried forward. The question however is whether the money retrieved is a gain or income?! If income you need to add it to your taxable income for this year. The reason it may not be a gain is that it is not an asset that has been disposed of. As it could not be calculated for gains purposes it would not fall into a black hole but must be accounted for somewhere, most logically as income. Sorry to add to your problem!