SUM a field with related record and update in another form

SUM a field with related record and update in another form

Hi,
I have one form called Record form for portal user and admin user to add record accordingly.
In this Record form, there is one Amount field.

There is another Profile form where each portal user will be prompted to create their Profile upon first login.
In this Profile form, there is one Balance field.

I want to SUM up the Amount field for each portal user and update it to the Balance field.

Please kindly assist.