Hi Khaireddine,
Here is how you go about doing it, works great.
On your item setup unit sequence group (eaBoxPL).. setup unit conversion 1 pl = 10 box, 1 box = 10 each, 1 pl = 100 ea
E.g. PO is 90 ea.
While receiving, use line item receiving, on first screen enter PO / item, qty as 9 and unit as Box. On next screen it will ask you for batch number / date.. enter batch / date. there are quantities and unit below, select 9 in qty and unit as box. click ok and it will complete the work. If this is not what you get, post some screenshots of your item setup, your item / qty / unit screen and what you have in batch/qty/unit screen.