We introduce an important physical effect, namely, the response of the dust particle charge to an oscillating electric field in a plasma sheath. The ac field may lower the average dust potential well below the corresponding floating potential U-f in a de sheath. The resulting dust potential will be called the dust de self-bias potential U-dc and is generally a sensitive function of the ac potential amplitude and of the ratio omega(ch)/omega between the so-called dust charging frequency omega(ch) and the ac frequency omega. The difference between U-dc and U-f maximizes for omega(ch)much less than omega and approaches 0 for omega(ch)much greater than omega. We propose that the dust de self-bias effect is important in all plasmas having a rapid potential variation and always has to be taken into account when calculating the charge and the force on dust particles in such plasmas. Since levitation of dust particles above surfaces normally requires negative dust particles, the dust de self-bias effect may explain why some experiments indicate that levitation of dust particles is easier in rf discharges than in de discharges.