The manor and its estate, estimated to be worth up to pounds 1m, were left to the trust by its former owner, Audrey Barrie-Brown, who died in September 1991.
A widow in her eighties, she had taken an interest in local archaeology late in life and her bequest was a windfall to a charity with previous assets of just pounds 11,000.
Shortly before her death Mrs Barrie-Brown had expressed the wish that the charity should use Fresden Manor as an educational and leisure facility for future generations to enjoy.
However, the charity's trustees have now put the house up for sale.
The codicil to Mrs Barrie- Brown's will, which simply left the property to the charity and was signed a few days before her death, contained no conditions.
The trustees' decision has led to fierce criticism and threats of legal action from some of the charity's supporters. However, Alan Bowman, the Oxford University historian who is chairman of the trustees, says the charity would risk losing money if it tried to run the house as an archaeological centre.
'We have had a full study of the whole question of whether it could be run on a financially prudent basis and the answer seems to be that it can't,' he said. 'There was no endowment to run it.'
Fresden Manor is also mortgaged, reducing its value to the trust to between pounds 600,000 and pounds 700,000.
The dispute highlights the problems that can arise when leaving money to charity.
'There is a great deal of difference between expressing a wish that a bequest be applied for a particular purpose and making legally binding conditions in the will,' said Bernard Sharpe, director of the legacy research company Smee and Ford.
But attempts to ensure that specific instructions are written into a will can be counterproductive. A charity that feels it cannot meet the conditions will normally have to turn down the bequest.
'Then, of course, it would fall back into the estate for distribution to other beneficiaries, perhaps the residuary legatees,' said Mr Sharpe.
'That is the irony - the legacy passes to somebody else, with no conditions attached. The charity doesn't get any benefit, and the intention of the individual to support that charity is lost.'
He said he had faced this situation several times during his previous employment with a charity working with handicapped people. 'On one occasion, we were left a small amount of money but had to undertake to maintain a grave in perpetuity. On balance, I didn't think we could take it up. The bequest went finally to another charity without any conditions.'
Money from legacies is enormously important to charities - it made up about a third of the voluntary income raised last year by the 200 largest fund-raising charitable organisations. However, only about one in three people in Britain make wills, and of these only about 16 per cent include charitable bequests.
So bequests could be worth even more to charities in future. Many have set up specialist units to advise those considering writing bequests into their wills.
'We have a pack both for the general public and for professional advisers, and we have solicitors on our staff,' said Michael Beaumont, head of the National Trust's legacy unit. 'Anything left to the National Trust is never used for administration but is used to buy and endow new properties or - perhaps more likely - to undertake major works at existing properties.'
'Charities should try to make sure in advance that onerous legally binding conditions are not put in,' said Mr Sharpe. He suggested that individuals considering making wills - or their solicitors - should contact the charity they want to support before drawing up the will.
The Law Society's annual attempt to encourage more people to write wills, 'Make a Will Week', runs this year from 18 October. Many participating solicitors offer a set- price service.
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