SB 8.22.2

śrī-balir uvāca
yady uttamaśloka bhavān mameritaṁ
 vaco vyalīkaṁ sura-varya manyate
karomy ṛtaṁ tan na bhavet pralambhanaṁ
 padaṁ tṛtīyaṁ kuru śīrṣṇi me nijam
Synonyms: 
śrī-baliḥ uvāca — Bali Mahārāja said; yadi — if; uttamaśloka — O Supreme Lord; bhavān — Your good self; mama — my; īritam — promised; vacaḥ — words; vyalīkam — false; sura-varya — O greatest of all suras (demigods); manyate — You think so; karomi — I shall make it; ṛtam — truth; tat — that (promise); na — not; bhavet — will become; pralambhanam — cheating; padam — step; tṛtīyam — the third; kuru — just do it; śīrṣṇi — on the head; me — my; nijam — Your lotus feet.
Translation: 
Bali Mahārāja said: O best Personality of Godhead, most worshipable for all the demigods, if You think that my promise has become false, I shall certainly rectify matters to make it truthful. I cannot allow my promise to be false. Please, therefore, place Your third lotus footstep on my head.
Purport: 

Bali Mahārāja could understand the pretense of Lord Vāmanadeva, who had taken the side of the demigods and come before him as a beggar. Although the Lord’s purpose was to cheat him, Bali Mahārāja took pleasure in understanding how the Lord will cheat His devotee to glorify the devotee’s position. It is said that God is good, and this is a fact. Whether He cheats or rewards, He is always good. Bali Mahārāja therefore addressed Him as Uttamaśloka. “Your Lordship,” he said, “You are always praised with the best of selected verses. On behalf of the demigods, You disguised Yourself to cheat me, saying that You wanted only three paces of land, but later You expanded Your body to such an extent that with two footsteps You covered the entire universe. Because You were working on behalf of Your devotees, You do not regard this as cheating. Never mind. I cannot be considered a devotee. Nonetheless, because although You are the husband of the goddess of fortune You have come to me to beg, I must satisfy You to the best of my ability. So please do not think that I wanted to cheat You; I must fulfill my promise. I still have my body. When I place my body for Your satisfaction, please put Your third step on my head.” Since the Lord had covered the entire universe with two steps, one might ask how Bali Mahārāja’s head could be sufficient for His third step? Bali Mahārāja, however, thought that the possessor of wealth must be greater than the possession. Therefore although the Lord had taken all his possessions, the head of Bali Mahārāja, the possessor, would provide adequate place for the Lord’s third step.