United want Telles, 27, to reinforce their left-back position but have yet to improve on their €15m bid.
Porto are showing off their best poker face and want United to exceed – or at least reach – the €20m mark.
Even with newcomer Zaidu showing promise at left-back, Porto know they will be losing a champion if Telles leaves.
Porto expect United to make a more generous offer in the coming days that will be closer to their requests.
The commissions (around €6m) have already been negotiated by United and Telles’ agent.
It has been reported that Telles already has an agreement for a five-year contract with United.
Should the deal go through, Porto will target another left-back with several names under discussion
Ruben Vinagre of Wolves and Rio Ave’s Matheus Reis are two players being considered by the Dragões.