"I would like to have everything cleared up early, but we cannot agree in July that I will be staying - that would be unrealistic," he told Sport Bild magazine.
"What happens when Chelsea or another top club comes along on August 21 and offers £24million?
"I would want to move, even at the very last minute of the transfer window."
"Hamburg made me a great offer last year and I turned it down because my aim is to move to a big club where I can improve and win trophies," added van der Vaart.
"My opinion will not change."