[AccessD] URGENT: Report Sum of the group shifted for one record

Cross posted Access-L AccessD
Please help me, I'm desperate :(
I want to sum the values from a text box in the Detail Section, and to put that sum in the text box in the Group Header.
In the group header is an unbound text box Text76 
In the Detail  section is the text box UkSalDug=IIf([txtSaldo]>0;[txtfld1]-[txtfld2];0) with Running Sum: Over All
In the code for Group Footer I want to pick this value with: 
Me.Text76 = Me.UkSalDug
But the Me.Text76 always get the sum of records of the previous group :(
So, I want:
John 100
40
50
10
Peter 200
120
80
Instead I get:
John 0
40
50
10
Peter 100
120
80
I can't do that in the report's record source, must be done directly at the report.
Can anyone offer a ray of light please?

Many TIA,

    Ervin

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