The Devil and Max Devlin

Max Devlin (Elliott Gould) is a shady landlord of a rundown tenement in Los Angeles who is rather jaded and callous towards his fellow man. One day while chasing an errant tenant, he is run over by a bus and killed. He descends into hell (which resembles a corporate business) and meets the Devil's chief henchman Barney Satin (read: Satan) (Bill Cosby). He is told of his life of sin and the fact that he is doomed to spend eternity at a section called Level 4. However, he is given a chance to save himself by convincing three other people to sell their souls in exchange for his. Max returns to Earth and begins his frantic quest with two months (ending on May 15) to complete his mission. Barney appears frequently throughout the movie to check up on Max's progress as well as both taunt and persuade him to carry out the plans. A running joke is that nobody, except Max, can see or hear Barney.
In addition to being temporarily alive, Max soon learns that he casts no reflection when he looks into any mirror. Barney tells Max that by a signed contract his soul belongs to him unless he completes his mission. Barney further explains to Max the conditions to get the three young people to sell their souls before the deadline and that Max will be given limited mystical powers which are called "magic property" to convince his three targets that they have special talents. The magic property lasts only as long as Max and the subjects are within sight of each other. Once Max completes his mission, his soul will be free and the three subjects will continue to live until the natural end of their lives.
Max's three targets are: Stella Summers (Julie Budd), a 19-year-old high school dropout and aspiring singer who has dreams to make it big; Nerve Nordlinger (David Knell), a 16-year-old high school geek whom has dreams to be popular by becoming a dirt motorbike champion racer; Toby Hart (Adam Rich) is an 11-year-old boy who dreams of having a father figure in his life in order to make his widowed mother, Penny (Susan Anspach), happy again. Max charms his way into each of their lives by landing a recording contract with Stella, trains Nerve into riding a motorbike after school for local races, and spends time around Toby while helping his mother operate a day care facility.
Along the way, Max discovers his innate decency towards all three of his subjects, the fact that he really wasn't so bad all along. He falls in love with Toby's mother and they plan to marry on the day that the deadline for Max is up. After Max receives another intimidating visit from Barney who demands that Max get the contracts signed as soon as possible, Max tries to get his three subjects to sign their contracts to sell their souls to Satin, but finds it more difficult then imagined. Stella refuses to sign her contract on the assumption that Max wants more than 20% of the profits he is currently receiving as her manager. Nerve is too focused on his motorbike training for an important race to notice. Toby refuses to sign his contract unless Max marries his mother.
Eventually, through various methods, Max does obtain all three signatures on the fatal contract (which, immediately after signing, prompts the good natured Stella, Nerve, and Toby to become angry and hostile... apparently sensing Max's hellish alliance). However, on Max's wedding day to Penny, right after he gets Toby's signature on the contract, Barney appears before Max and tells him he will take the three chosen ones at the stroke of midnight (having lied to Max earlier about letting them live natural lives), while Max gets to live until the natural end of his own life before going back to Hell. Max is horrified and enraged by this and prepares to tear up the contracts. In the film's most intense scene, Max is transported to Hell where Barney appears before him in full devil regalia and screams at Max of his terrible fate of torment in Hell if he burns the contracts. Max does so anyway, and is immediately transported back to Earth.
At first, Max thinks that he's just doomed himself to be sent to Hell at the stroke of midnight. He then leaves his own wedding reception to say goodbye to Nerve, then Stella and finally Toby (all of whom are on friendly terms with him once again). Upon returning to Penny's house to stay goodbye to her as the clock ticks towards midnight, he suddenly realizes he is living again when he sees his reflection in a mirror. Max is overjoyed and he figures out that his one, kind, unselfish act to sacrifice himself for his three victims has deemed him unfit for Hell.
The last scene shows Max, Penny, and Toby attending a concert that Stella is giving which she claims is her "farewell concert" to find herself. After she sings a new song in flawless tone without any magic talent, Max is seen looking upward (as a reference to Heaven) and mouthing "Thank you very much".

Question: Who was performing at the concert?
Answer: Stella