Col F contains a purchase amount including a GST tax, col G contains the same
purchase amount excluding GST. Col K is sale price inc GST, col L excluding
GST.
When ever the value in L is than that of G, I need to calculate the
difference between the two amounts of tax, ($2000 at purchase & $19090.91 at
sale, being $90.91) I then need to show this figure in col I as well as
adding it into col H.

F G H I
K L
$22000.00 $20000.00 $861.91 $90.91 $21000.00
$19090.91

Cheers from frustrated excel novice, Paul