"They wanted to come over and have a personal chat," Dufner told Bild. "This is what we have done, they were welcome.
"We have not reached an agreement. As far as I am concerned, Stoke feel we will automatically agree to a transfer at some point, as the contract expiry date is just 12 months off - but we do not plan to do so.
"I feel they will now consult, and then we will see whether we receive a written offer or not. We would think about that if Stoke agree to what we have told them."
It is reported that Diouf, who scored 12 league goals last season, could cost up to €8m (£6.9m).