SAP Message VPK1321 - The +/- sign must not be changed

Diagnosis
In resource-related billing, you have entered an expenditure to bebilled with a sign different from that for the open expenditure.
In the sales pricing for project or when generating a quotation forservice orders, you have entered a transfer expenditure with a signdifferent from that for the planned expenditure.

System Response
The expenditure to be billed, or the transfer expenditure, has beenzeroed out.