National Highways Authority Of India [NHAI] — Sriperumbudur toll plaza

On 13th nov 2021 I crossed sriperumbathur toll around 7.13am from chennai towards kanchipuram.I was charged Rs. 50 through fasttag. Then from kanchipuram to chennai i crossed sriperumbathur toll around 3.55pm on the same day i was charged Rs. 50 through fasttag instead of charging the balance amount for return journey. I don't know what the fasttag purpose. I request the NHAI to take action against the toll plaza.