Customization in Purchase order

I added a custom link field “Department” in Purchase order and custom select field “Billing Type” in purchase order item. “I need to show the ‘Billing Type’ field in the Purchase Order Item when the ‘Department’ in the parent Purchase Order is set to ‘Hiring’.”

.How handle it?

Elaborate a bit more please

When Department is hiring I have to give billing type as hour/month/day. For other departments ,providing a billing type is unnecessary.