Originally Posted by
Arasilion
That Valar had no power at all to put aside the fate of Man. What happened is that Mandos felt pity for Beren and Luthien, so he forwarded their case to Manwe who had direct communication with Iluvatar, the only Being who can put aside the doom of mankind. Iluvatar granted their wish, but with the condition that Beren stay in Valinor to live forever and never return to Middle-Earth, or that both should come back to life in Beleriand and live to die another mortal death.