Oh, this is an interesting one - and may not be easy. I think you may find SharePoint workflow to be your simplest solution - you could have a workflow on the library for your Request form and when it is submitted, go update the item in the Inventory library.
However, since you just want to update one item in the other library, you may be able to do this with CAML. Is the Inventory form submitting to a SharePoint Library? Or is it a List?
Here's a blog post that may get you started....