He is depicted as Bali's doorkeeper in this serial I am watching about Ganesa. Which Purana says this and was that as atonement for Vamana's trickery?
The Devi Bhagavata Purana actually says that. Might be there in other Puranas as well.
17-20. Visnu, the Lord of the Universe, assumed His Dwarf (Vâmana) Incarnation with the express object to cheat Vali, the king of the Daityas. Now, O Muni! The king Bali, performed one hundred sacrifices; he was the protector of the Vedas, virtuous, charitable, truthful and self controlled; why was such a man dislodged from his position by Visnu, the Powerful. Who was victorious in this affair? Was he the Vali, who was cheated? Or was it Vâmana Deva, the expert in making nice pretence? Who was the better of the two? I have got grave doubts on this point. O the best of the twiceborn! You are the composer of the Purânas, virtuous, and liberal hearted. Speak what is true (and thus tranquil my heart). 21-23. Vyâsa said :-- O king! The victory was certainly Bali's, in as much as he fulfilled his promise and gave over his kingdom of earth to Visnu. And in as much as Visnu in his 5th or dwarf Incarnation deceived Bali, he had to become a dwarf (i. e. a small mean person indicated even by the shortness of his body). O king! There is nothing superior in religion to truth. See! S’rî Hari even had to become, for his falsehood, a gate keeper of Vali. O king! It is hardly possible for a human being to observe in every way the injunctions of truth. 24. Powerful, indeed, is Mâyâ, composed of the three qualities and of various forms. By Her is created this Universe, made manifold by the admixture of the three qualities (Sattwa, Rajas and Tamas)
From the Purana's 4th book's 4th chapter titled "Adharma"
Yes, Lord Vishnu becomes a door-Keeper of Bali. It referenced in Dashavatar of Lord Vishnu.
When king Bali had defeated by the Lord Vamana. Later lord Vamana sent the Bali to the Hartal folk(पाताल लोक) and Lord Vishnu Became his door-keeper.
This is not a boon but it was cursed for Bali. Because of this, Bali could not back to his kingdom.
When Bali ruled in his kingdom at that time he breaks the all social rules like the education system, Political system etc.
After He defeated by Lord Vamana, Then He sent to him Hartal folk(पाताल लोक) with giving lots of wealth.
Lord Vamana wanted that Bali would not come back in his kingdom. So, He becomes a door-keeper of Bali.