I am designing a system on SharePoint. There is a approval list for the items. The members can approve, reject and edit the items. One from approval list has to fill the "assigned to" field in the item while approving it. The user who is added to "assigned to" field should able to edit the content of the item after it is approved. So, how can I give the edit permission to the users after they are added assigned to field of a specific item?

The situation is:

approval list: A, B ,C (edit, view permission)

users: x,y,z .... (no permission, view after approval)

items: item1, item2, item3....

items are invisible. A approved the item1 and added X to "assigned to" field. It means This item is under X's responsibility. But X hasn't got edit permission. we can't give edit permission to X for every item. He should edit the items after he is written into the "assigned to" field.

How can I create this workflow in SharePoint? Please urgent help needed.