Blanket service PO

I need to create a blanket service PO(no quantity) of 12000$ and need to release 1000$ amount every month as an when i receive the service.The blanket PO should be open and should show the remaining open amount till the time whole service is not rendered.
Do anybody has any document wherein I can see the setups?I am clear with setups related to the releasing of the stock blanket PO having quantities.

Please help

Thanks