I would like to know how I can sum up fields in different records (get a number in a separate field) that contain identical information (like ID's). I don't want to have the outcome in a report, but in a unique field on a layout

The idea is to create a detailed invoice with a list of different working hours.

Create a self-relationship (ID to ID).

Create a calc field that is the sum of the related amounts [sum(ID::Amount)].

It won't show a "running total", but it will show an accurate total even if not all records are in the current "found set".