However, if I select a specific date of this week (through a filter) it shows me in the KPI 'sales of this week' also only the sales of this one selected day. How can I fix, that the KPI always shows 'sales of this week', no matter what date I choose?
Set Analysis is what you need, if you share your expression and fields that may be filtered to change it, someone could modify your KPI expression to so what you need
It would be much easier to answer if you shared the expression with us. However I guess that you will to add a SET modifier into your expression to ignore your selected date value.
If I assume that you already have a SET in your expression then the modifier would only be YourDateField=
If you don't already have a SET then you would need to add {< YourDateField= >}
Your final expression would look like this SUM({< YourDateField= >} yourExpression)
