Sec 139(8A) d return

If a person pay self assessment tax Rs. 50,000/- in month of December' 23 but he didn't file his return till December. If he files his return u/s 139(8A) in January' 2024 and his tax liability comes Rs. 70,000/-
Now my question is that whether penalty of 25% will be charge on remaining amount i.e. Rs. 20,000/- or on whole amount Rs. 70,000/-