Please refer to this thread Unreserving Parts on a customer Order

Hi @Nagah.khaled 

You can follow the below steps,

1. RMB on the line and select “Reservation Details”

2. Then RMB on the line and select “Reserve Manually”

3. 

   Then in the dialog box under Qty to reserve field enter the qty in negative. for eg if 10 qty is to be unreserved then enter -10 and press ok. Make sure you select the correct location and other information right.

4. 

in some cases system raised this message

what shall i do

Hi @Nagah.khaled 

This has to be investigated technically. Hence I suggest you to raise a ticket to IFS Support.

Best Regards,
Tharaka.

Why would it need investigated technically?  The error indicates there is a Custom Event setup to validate the quantity of the part based on the W/D/R value and possibly a custom field on the requisition.  

@Nagah.khaled - you would need to understand the intent of the custom event and ensure you are complying with the requirements of the event when unreserving that item.