The Daily Mail says the Brazil international has an £80million release clause written into his contract - but the Portuguese giants will let him leave for £38m.
Talks between both clubs will accelerate in the coming days after Hulk's Olympics involvement came to an end at the weekend.
But Porto's valuation is threatening to scupper the transfer despite Hulk's attempts to force through a move to Stamford Bridge.
"I have left the door open to be transferred," said Hulk.
"My preference is to play in England or Spain because they are the most watched league in the world. Now is a great time for me to move."