National Australia Bank yesterday said it decided to write down to nil the carrying value of $309 million in receivables considered due from the Australian Taxation Office. NAB said this would not affect cash earnings.The amount relates to the 50 per cent payment of disputed tax over interest deductions claimed by NAB over hybrid securities (exchangeable capital units, or ExCap) originally sold in 1997.The ATO considers that NAB owes it another $309 million, though NAB is unlikely to pay this quickly given its position (reiterated in its ASX announcement yesterday) that it would appeal any adverse court decisions.NAB said yesterday it follows further analysis of the Full Federal Court's decision in a similar case involving hybrids from St George Bank and which "may be unfavourable to NAB on the issue of whether interest payments on the ExCaps are generally deductible."