sumif value using check box

i am using checkbox for the sum calculation.
=sumif($I$5:$I$5,TRUE,SUM($G$5-$K$5)

3 G, I, J, K, L,
4 payment, T/F, checkbox, down payment, outstanding
5 $25,028.95, FALSE, (unchecked), $(7,508.69), (need formula here)

When i check in J5 the K5 must be deducted from G5 and the sum must be calculated in L5

Assuming you want L5 blank until the checkbox is activated...

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However, you don't even need a checkbox for this, you can just test if K5 is not blank.