Populate dropdown conditionally on runtime in sharepoint

Hi,

I have two list in share point 2010

1- Inventory list

columns: Item name, In Stock(boolean)

2- Inventory Issuance

columns: Transaction type(issue, receive), Item, employ, date

There are four field on my Inventory Issuance form 

I want Item dropdown to be populate on user selection on Transaction type like 

If user selects transaction type = issue -> dropdown should populate all items from Inventory list having In Stock=true

If user selects transaction type = receive -> dropdown should populate all items from Inventory list having In Stock=false

Scenario is something like dropdown will be populated on runtime on user selection of transaction type column.

Please help me out.

Thanks