To lie to the Holy Ghost. The sin is here pointed out. The lie was not to men, but to God. The apostles were moved by the Holy Spirit. These verses show clearly that the sin was hypocrisy and deception in the name of religion. These verses also show that the gifts of goods were purely voluntary.
(PNT)

Thou hast not lied unto men, but unto God: in that lying unto God is so often charged, and no express mention is made of Ananias’s vow, some excuse him of sacrilege, and charge him the more deeply with ambition, covetousness, lying, and hypocrisy, to the apostles, whom he intended to deceive.
(POOLE)
Acts 5:4:
#Ac 5:4| (4) While it remained, was it not your own? And after it was sold, was it not in your own control? Why hast thou put this thing in thy heart? Thou hast not lied to men, but to God. Here Peter brings together the influence of Satan, and the free agency of the tempted, just as he had, in former discourses, the free agency of men, and the purposes of God (see TFG "Ac 3:17" and see TFG "Ac 3:18"). He demands of Ananias, "Why has Satan filled thy heart to lie to the Holy Spirit," and, in the same breath, "Why hast thou put this thing in thy heart?" The existence and agency of the tempter are distinctly recognized, yet it is not Satan, but Ananias who is rebuked; and he is rebuked for doing the very thing that Satan had done, showing that he is as guilty as though Satan had no existence. Indeed, he is rebuked for what Satan had done. The justice of this is manifest from the fact that Satan had no power to fill his heart with evil, without his co-operation. That he had rendered this co-operation, threw the responsibility upon himself. Peter's knowledge of the deception was the result not of human information, but of the insight imparted to him by the Holy Spirit. This is necessary to the significance of the entire incident, as well as to the purport of Peter's own words.
(OCA 68)
(The Fourfold Gospel and Commentary on Acts of Apostles