Limit editing to specific fields in a SharePoint 2010 list

I am wanting to allow limited editing of a SharePoint 2010 list item, to allow only editing of a few fields, depending on data in another sharepoint list. I have two lists:

Stock
=====
Stock Item   Maximum Stock Level   Current Stock Level
----------   -------------------   -------------------
My Item      100                   10
My Item 2    200                   25

Permissions
===========
Stock Item (lookup)    Users (person, multiple)
-------------------    ------------------------
My Item                User 1
My Item 2              User 2

When 'User 1' clicks on My Item, 'Edit Item' is disabled (since they are in a custom group 'Stock Updaters'). I then want another icon in the ribbon 'Update Stock', which they click on and a form shows that allows them to edit the current stock level only. When they view 'My Item 2', either 'Update Stock' is disabled, or it just shows a view stating they don't have access to update the data.

How can I implement this, us开发者_运维知识库ing InfoPath 2010? If they need to have contribute rights to update the data, is there a way of preventing this (unless they are an admin), or the prevent the edit form being shown (perhaps through workflows, showing a different form instead)?

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Have you explored custom list forms? That will give you control to execute custom rules and enable/disable fields based on whatever rules you have.