Correspondent Michael Schmidt said the transaction is not quite as simple as earlier thought, involving bringing Felipe Massa back out of retirement and moving Bottas to Mercedes.
He reports that Mercedes and Williams are yet to agree a compensation amount for the Bottas deal.
"Mercedes has made it clear to Williams that they are not willing to pay any sum," Schmidt said.
So it's here that the Paddy Lowe factor could come into play, amid reports the Briton could become Williams' new team boss. But he remains under contract at Mercedes until May.
"We hear that Lowe will not take the position of technical director (at Williams), but that of team boss," said Schmidt.
"Claire Williams would then take over from her father. Supposedly, Lowe also wants team shares," the Auto Motor und Sport correspondent added.
But if the Lowe deal does not work out, the Williams-Mercedes deal regarding Bottas may also falter.
If that happens, Pascal Wehrlein's expected move to Sauber will be halted.
"Mercedes decided not to take a chance with its young drivers," former F1 driver Vitaly Petrov told Russian radio Sport FM.
"They had the option with Pascal Wehrlein, but why they did not take the risk, no one knows.
"But Valtteri Bottas is a good option for the team. It will not be easy to fight with (Lewis) Hamilton, and if the cars had remained the same it would be even harder.
"But with the new regulations, a new car, Bottas has a good chance of performing on par with Lewis," Petrov added.