Chinese side Shanghai Shenhua have warned PSV Eindhoven that they will have to pay market value for Sun Xiang and Sun Ji.
The duo arrived in Eindhoven at the start of the week to undergo a trial with Ronald Koeman's side.
Shenhua are happy for their players to showcase their talents in Europe, but will not be forced into a cut-price sale.
"We have no problems with the trial," Shenhua's Wu Jinan told the Shanghai Evening News. "But things will have to be negotiated properly. They are part of our future plans."
John de Jong, meanwhile, returned to training in midweek but admits he faces a long road to fitness.
The midfielder has not played for PSV since March 2005, but his knee would appear to be on the mend and he is looking to step up his rehabilitation.
"The turning is still difficult and you cannot be too optimistic when you have been out for so long," De Jong told PSV's official website.