I'm not aware of a supported method for shrinking a database device.
How much of the device has been allocated to databases, ie, do you have 180GB of free space on the device? (sp_helpdevice <device_name>)
Your easiest option will likely be:
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1 - reverse engineer the 'create/alter database' commands for the database(s) that resides on the device in question
2 - shutdown applications that access the database(s) in question
3 - perform a complete/full database dump of the database(s) that resides on the device in question
4 - drop the database(s) that you dumped in step #3
5 - drop the device in question (afterwards you may need to explicitly remove the physical file at the OS level)
6 - create the new 120GB device (disk init)
7 - create the database(s) with the 'create/alter database' commands from step #1
8 - load your database(s) from the dump file created in step #3
9 - online the database(s)
10 - restart applications that access the newly recreated/reloaded database(s)
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