How can be the product cost goes minus in daily and monthly sales reports ?
Shouldn't the cost must be a positive number ?
Yes, it shouldn't suppose to be -ve. What is you inventory accounting method?
If you can provide step to reproduce then it will be easier for us to find the cause.
What do you mean by reproduce ?
Please take care not to loss, damage or change the data.
Please give us a report and guidance to solve the issue ?
by reproduce means how can I replicate or get the same issue/error on the demo. If you can give step that will help us rectify the issue otherwise it will be quite difficult. Your site is not access-able here, error 1020.
I don't know the cause yet just suspecting that you are changing the inventory accounting method that could cause such issue.