Able to issue the serial number even it is at vendor stock (subcontact)

Hi,

We want to stop our warehouse guy posting goods issues if he enters a serial number which is at vendor stock (subcontact). We suggested the selection by "Storage Location <> Blank", which is the way to exclude the stock at vendor's site (stock at vendor has storage location blank) but still the warehouse guy can enter the serial number manually and mistakely.

is there a standard way to do it? if not, which user exit will be the best option if we want do it through coding?

Thanks very much for the help.

Regards,

Jully