An estimated 2,000 more people will qualify for a windfall from the sale of Irish Nationwide if the building society is not sold until next year.

Some 100,000 members of Irish Nationwide will each qualify for a payment of about €10,000 from the sale of the building society, which has been valued at between €1 billion and €1.5 billion.

Irish Nationwide customers with qualifying share accounts are only eligible to be treated as members - and therefore entitled to a payout from the sale of the building society - if their account's balance is above €125 for the last six months of the financial year before a sale and up until the sale.

If a sale is concluded this year, a customer holding a share account with a balance of more than €125 since July 1st 2007 would not qualify for a windfall. However, the same customer would be in a line for a payout if the sale was not completed until January or later next year.

It is understood that about 2,000 customers could fall into this category and earn a windfall if the sale is delayed because they have increased the balance in their accounts. "There were people who didn't keep an eye on their balance," said one Irish Nationwide source who has been handling customers queries.

Some customers missed out on eligibility by increasing the balances in share accounts last year. The new building society legislation paving the way for a trade sale was passed in July 2006 but some members failed to increase their balances until August, making them ineligible for a payout from a sale in 2007.

However, many have subsequently increased their balances and could yet qualify if the sale is not completed until next year, according to Irish Nationwide sources.

The society requires a minimum lodgement of €20,000 to open a new share account. However, members can and have reduced the balance in this account to below €125.

The building society is expected to clarify the exact criteria for eligibility on a payout from a sale when the demutualisation documentation is published.

The society has tidied up its membership records as part of an audit in advance of a sale. It has been checking to ensure that members are not listed several times in a process known as "de-duplication". This will prevent members receiving multiple payments from sale proceeds.