Error: You are not allowed to have negative actual costs

Error Number = 6470  Stored Procedure= taPAEmpExpenseLineInsert  Error Description = You are not allowed to have negative actual costs

Run a query to update the PostedTotalCost to 0.0 for the project and cost category in question:

update PA01301 set PAPostedTotalCostN = 0.0 where PAPROJNUMBER = 'PROJECT123' and PACOSTCATID = 'CATEGORY123'