Deutsche Post insists its sale of part of Postbank will go ahead in spite of a sharp drop in Postbank's share price, which is making the terms of the deal much worse for Deutsche Bank (NYSE: DB - News) , the acquirer.

Deutsche, Germany's largest bank, agreed last month to pay €2.8bn ($3.8bn) for just under 30 per cent of Postbank. Subsequent large sell-offs in stock markets have cut Postbank's market capitalisation to €3.4bn.

"There are no changes to the deal," said Deutsche Post. Deutsche Bank said it would not comment on market rumours on Thursday that the deal could be scrapped or renegotiated.

Other people with knowledge of the deal said they believed it would proceed, in spite of the gap between Deutsche's offer and the current value of Postbank.

The deal was announced a month ago, on the last trading day before the collapse of Lehman Brothers deepened the financial crisis.

However, an investment banker not involved in the deal said: "You would be surprised if they don't go back [to Deutsche Post] and say, 'Hello - things have changed here'."

The sale of the minority stake in Postbank was the second big deal in weeks in German banking after Allianz, the insurer, agreed to sell Dresdner Bank to Commerzbank in a two-stage process involving cash and shares.

Allianz and Commerzbank have also maintained that their deal - which will see the insurer become the bank's largest shareholder - is on track in spite of sharp changes to the valuation of Dresdner assumed in the transaction. Both deals are set to be completed in the first quarter of next year.

Deutsche Bank bought Postbank, which has Germany's largest retail customer base, in part to compensate for a downturn in investment banking, the main source of its profits.

It also wanted to keep competitors - such as Santander of Spain - out of its home market. A person familiar with the deal said: "As long as Deutsche has the stake it shuts others out of Postbank."

Another person said Deutsche would be reluctant to try to renegotiate a deal that also implicitly involves the German government, which owns about 31 per cent of Deutsche Post.

Deutsche Post also has an option, first likely to be available in 2010 or 2011, to sell its remaining Postbank stake to Deutsche Bank at €42.80 per share. Postbank's shares closed down 8.4 per cent at €20.69. Deutsche Post fell 14.1 per cent to €9.97, while Deutsche Bank fell 6 per cent to €31.87.