Warning message in PO for One time vendor

Hi,

I want to put a warning message in PO to give a message to user for One time vendor.

In my Company there is amount limitation for purchasing through one time vendor. It should not be more than 5000 in single PO.

For that I want an error / warning message to flash at the time of PO saving. Please let me know how to configure the same, as there is no standard message available for this.

Thanks,

Ranjan Kumar