Bayern working on Martinez deal

But with the deadline falling at the end of August, the Bavarian giant faces a race against time to sign the 23-year-old on a five-year deal.

Rummenigge explained that the buy-out clause in Martinez's deal stipulates he can only leave Bilbao for a fixed sum of 40 million euros ($47.68 million).

"It's a very complicated clause, so the whole subject would cause certain legal and tax problems," added the Bayern chairman.

The Bayern boss said Athletic Bilbao was showing little cooperation, but added: "We're working on it and I hope we can announce a deal by the end of August. But it's not entirely easy."

Rummenigge has revealed Martinez is eager to join Bayern, which starts its league campaign at Bavarian neighbour Greuther Furth this weekend, and Munich is hoping to get its man for less than the asking price.

"Due to the fact he's prepared to forego a great deal of money in return for the five-year contract we intend him to sign here, he's indirectly contributing to the transfer fee," added Rummenigge.

"It definitely means the total investment would be significantly reduced."

But Bayern president Uli Hoeness admits he is frustrated by the lack of cooperation from Bilbao.

"At the moment, it doesn't look like the transfer will work out, because the people in Bilbao are very, very stubborn and are obviously doing all they can to prevent the transfer," he told German television channel NTV.

"If my colleague, the chairman of Bilbao, is willing to make a transfer agreement, he could get 40 million euros fairly quickly."