When shares are forfeited, the amount already received is credited to:

NCERT (2026-27 edition):
Forfeiture of Shares – “… It may be noted here that when the shares are forfeited, all entries relating to the forfeited shares must be reversed except the entry relating to share premium received, if any. Accordingly, the share capital is debited to the extent to called-up capital and credited to (i) respective unpaid calls account i.e., calls in arrears and (ii) share forfeiture account with the amount already received on shares.”