How do you deal with POs that you don't intend to receive fully?

I would like opinion of our user community on how they deal with POs that they don't intend to receive fully. For example, the PO is for $100 and you have received goods worth $30 against it. You don't want to receive the rest of it. Do you change the PO to decrease the amount to $30 (suppliers would get the changed PO automatically)? Or do you just call up supplier and ask him to not ship anything more and leave the PO as it is?

When would you release excess budget reserved against this PO (it can potentially release $70 back to your budget pool)?

Thanks,

Deepak Sanghi