Available Quantity Incorrect

Something has gone wrong with a PO. 

Firstly, an item is showing stock available Qty 2000, but when you click in the details, it shows there should only be 314. 

The other item on this PO has an error in it's listing as well. When I try to process the inv, it tells me the invoice number is a duplicate. I found the Inv no was processed and then a Reversal was done. Not sure how this reversal is done and I can't delete it, it wants to shut the system down. 

Any suggestions?

Thanks

Nicole