SSRS Sum issues in Group footer

I'm working on SSRS 2005

Im having an issue with Sum of a report group in the report footer to get my grand totals. This report is grouped by Vendor. Some vendors we have credits with but an over all balance. Other vendors we have an over all credit with. I want to show both but dont want to sum the ones with an over all credit. This is so our totals will come out right but we can see who we need to collect these credits from. In the example below I dont want to sum any amount from Vendor D. Vendor A has a sum of 0 dollar so its OK. In the below example I would like to see a grand total of $105.00 not $95.00 which is what I get if I just sum my net amount due.

Example:

Vendor A = $0.00 (This is my Net amount due for Vendor A)
($100.00)
$100.00

Vendor B = $25.00
($50.00)
$75.00

Vendor C = $80.00
$80.00

Vendor D = ($10.00)
($65.00)
$55.00

I have created the below expression in the group total so that Vendor D will show 0 dollar in hops I could just sum the textbox with group totals.
=(iif(sum (Fields!net_amnt_due.Value)<=0,0,sum (Fields!net_amnt_due.Value)))
Below is the expression I have in my report footer textbox.
=Sum("textbox43")
It says that the value expression for the Report Footer uses a numeric aggregate function on the data that is not numeric.

Any ideas will be helful as I've been trying different things over the past weekend with no luck.