Decrease the Provisioned Size of a VM in VS4

Hello!,

I have VS Enterprise with four ESX 4, one of my VM´s (VM version7) have one Virtual Disk of 100GB, Type Thin, and using 40 GB, I want to change the Provisioned Size from 100GB to 50GB whith the VM powerred off,  but only allow me to increase the size, not decrease.

Any suggestion?

Thanks!