Heidfeld, Fisi, still waiting on 2008 deals
Recent reports said that a contract for 2008 was being readied by lawyers, but team boss Theissen said the 30-year-old might have to wait until after the looming summer break.
"We have had some very good discussions but still nothing is signed," Theissen said.
He added: "No-one should expect a conclusion this weekend — it will probably happen only in the summer break."
The announcement on Thursday that former BMW-Sauber test driver Sebastian Vettel will drive for Toro Rosso in 2008, however, increases the likelihood that BMW will field an unchanged race lineup next year.
34-year-old Renault driver Giancarlo Fisichella, meanwhile, told reporters at the Hungaroring on Thursday that he is "expecting" to be given a new contract for 2008.
Team boss Flavio Briatore has reportedly delayed his decision about drivers for next year, while it is a fact that tester Nelson Piquet Junior is desperate to make his grand prix debut in 2008.
Fisichella said in Budapest: "I'm doing my best and working really hard and for that I'm expecting once again to be confirmed for next year.
"Soon we'll know — but I don't know when.
"I'm happy. On this aspect, everything is under control and I'm quite confident. I've done my best but unfortunately the car hasn't been so good."