Hi Rajesh,
If you are facing issues in the standard FM in event 120, then I would suggest you to place a clearing lock against the child invoice, at the time when that invoice gets disputed.
So, if the clearing lock is there, it will not clear the invoice the child level and will sit at the child level as payment on account.
Once the dispute is sorted out, you can remove the clearing lock and the payment and the invoice gets cleared in the Account maintenance\auto clearing run.
Moreover, you can automate the functionality of placing\removing the clearing locks in the dispute process as per your business rules.
Thanks,
Amlan