Unit Costing

We have a client who uses FIFO. They are selling, and returning goods before any purchase invoice has been processed, these are processed at the end of the month via an import. The problem we are having is that because we are using FIFO, these sales are being applied against these returns, which have no unit cost. This is leading to stock valuation entries, albeit only at a date in time, it does eventually wash through with the Adjust Cost Item Entries.

Does anyone have any suggestions how to avoid this?