A negative in Open Items can be causes by several things:
1.A debit memo was created to correct a grant, but it wasn't linked properly to the voucher.
2.A debit memo was created when voiding a check as well as when doing a grant adjustment and both were posted.
3.A voucher was created with negative line items.

How to fix each situation depends on what the circumstances were. Usually the way to fix these situations are:
For #1: Click the Create voucher Item button on the Debit Memo.  Typically the date chosen would be the original date.  Edit and post the voucher from the Item Entry tab.
For #2: Click the Create Debit memo button on the negative item.  Typically the date chosen would be the original date.  Edit and post the voucher from the Item Entry tab.
For #3: Click the Create Debit memo button on the negative item.  Typically the date chosen would be the original date.  Edit and post the voucher from the Item Entry tab.