Sales Order and unavailable Quantity

Dear All
Let say I have item "BIKE" with available = 5 EA with stocking type = "S"
I made Sales Order for "BIKE" with quantity 8 EA, and automatic SO create 2 line
First line "BIKE" with 5 EA
Second line "BIKE" with 3 EA, cancel

My Question :
1. How to make sales Order still on 1 line, ignore available quantity and not cancel it
2. For Unavailable Quanity (3 EA), how to make it automatic create Purchase Order or anything process for made that item available (produce or buy to suplier)

Hope somebody can help me for this, Thanks

Regards

Juned