Śrīmad Bhāgavatam 2.7.17 presents an interesting conversation. There, Brahmā says that Vāmana claimed the entire world from Bali using three “deceptive steps” (tripada-cchalena). This is said in the third line of the śloka. The fourth line answers the anticipated question, “How can Viṣṇu’s avatāra, Vāmana, employ deceit, an immoral tactic?”
The answer Brahmā submits is that it was not an immoral deceit because Vāmana begged (asked) the three steps of land from Bali, and Bali (very) willingly gave it. If someone gives you something willingly, it is not theft. There was some deceit involved because Vāmana literally asked for three steps (tri-pada), which on the surface literally means “three paces of land” – but which he intended to mean tri-pada – the three sections of the world: heaven, earth, and the netherworld.
Vāmana’s wordplay was deceptive, but the fact that he openly asked it from Bali, who did not request further clarification and willingly gave it, absolves Vāmana from immorality.
Actually, the case can be argued further. Bali’s guru, Śukra, told him that Vāmana was using words deceptively and would take the three divisions of the world by his “three steps.” Bali therefore was quite aware, and still very willingly granted the request to Vāmana.
Remember that Bali is the grandson of Prahlāda, and therefore is a Vaiṣṇava, despite living in netherworldly external trappings.