You could make use of a helper column.
Fill down column D with =ABS(C1), and change you formula to:
=MAX(SUMIF($A:$A,LEFT($A283,5)&"*@*",D:D),SUMIF($A :$A,$A283,D:D))

"Dallman Ross" wrote:

I have the following formula:

=MAX(SUMIF($A:$A,LEFT($A283,5)&"*@*",C:C),SUMIF($A :$A,$A283,C:C))

The trouble is, some numbers in C are negative. I want to add
them up too, or rather their absolute values.

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dman