The British Museum (BM) has denied that it tried to persuade a German university not to return a fragment of the Parthenon sculptures to Athens.
The Greek government hopes the return of a heel will strengthen its hand
Campaigners for the return of the BM's Parthenon sculptures - the Elgin Marbles - say the museum phoned Heidelberg University over the return.
The BM says a call was made - but only to request information about the move.
Greece's culture minister received the fragment - an 11x8cm carving of a man's heel - in Heidelberg last month.
In return it will receive the head of a small statue from the collection of a museum in Greece, says the British Committee for the Reunification of the Parthenon Marbles.
The BCRPM alleged that the museum "tried (too late) to persuade the university to go back on its undertaking to return the piece".
But a BM spokeswoman said the phone call to Heidelberg "was in no way an attempt to persuade them not to return the heel" and was simply intended to find out about the planned timing of the handover.
The return of the fragment has not changed the museum's position regarding the Parthenon sculptures in its collection, said the BM's spokeswoman.
The Greek culture minister receives the heel in Germany (University of Heidelberg)
The Greek government hopes the handover will boost its case for the return of the Parthenon marbles in the British Museum.
The heel is from one of the slabs of the frieze which ran round the top of the 2,400-year-old Parthenon temple, depicting a religious procession in ancient Athens.
Accepting the heel from university authorities, culture minister Giorgios Voulgarakis said: "Today is of historic importance. For the first time in some 200 years, a valuable fragment of the Parthenon from abroad is to be reunited with the monument."
He praised Heidelberg for not being part of the "tacit pact of museums such as London, Paris, Vienna, Rome, Palermo, Copenhagen, Munich or Wuerzburg, which hold Parthenon sculptures".
"The obligation to restore the Parthenon sculptures is an obligation of the whole of civilised humanity," he said.
The University's former Vice-rector Angelos Chaniotis said one reason the university's move was significant was "its possible consequences for the Parthenon sculptures in the British Museum".
The British Museum wants to keep its Parthenon marbles
Following the report that the head of a sculpture would be sent to Heidelberg in return for the heel, Mr Chaniotis told the BBC News website: "There has been no permanent loan from Greece to Heidelberg yet. There is a piece under discussion, but it will certainly take several months until a decision is taken."
The Greek government has for years campaigned for the return of the British Museum's Parthenon marbles - most of the surviving parts of the frieze and other sculptures.
These were removed by British envoy Lord Elgin at the beginning of the 19th Century.
The museum says it is not at liberty to return the sculptures, and believes they are well looked after and available for millions of visitors to see in London.