Costing issue with credits

We manufacture an item at a cost of say $3000.00. We can see the part go into inventory at the correct cost. We issue sales order and invoice. We can see the cost coming out of inventory correctly. Curtomer returns item for what ever reason and we issue credit against the sales order. Item goes back into inventory at different cost. Sometimes it's higher than original cost, sometimes lower. Should it not go back into inventory at original cost when credit is issued and posted? Trying to get a handle on this.

Thanks