col1 col2
100  200   <- known 100 and 200
300  400   <- known 400 and 600
---------  <- LaTeX decides to page break
500  600   <- known 900 and 1200

col1 col2
 100  200
 300  400
[400][600]  <- use known
-----------
[400][600]
 500  600
{900}{1200}
 {sum}

documentclass[a4paper]{scrartcl}
usepackage{longtable}
begin{document}
newcommand{uebertrag1}{0}
newcommand{uebertrag2}{0}
newcommand{summe}{0}
begin{longtable}{cc}
col1 & col2\endfirsthead
col1 & col2\
Übertrag: uebertrag1 & uebertrag2\endhead    %<- dynamic binding?
Übertrag: uebertrag1 & uebertrag2\endfoot
gesamt: uebertrag1 & uebertrag2\
Summe: summe\endlastfoot

renewcommand{uebertrag1}{100}
renewcommand{uebertrag2}{200}
renewcommand{summe}{300}

100 & 200 \newpage

renewcommand{uebertrag1}{400}
renewcommand{uebertrag2}{600}
renewcommand{summe}{1000}

300 & 400 \

end{longtable}
end{document}