Creating Assembly Order in relate to Job module

Guys,

My customer has this scenario. They have projects, and they planning to use the job module as they can track the expense and cost for the project by phases when they need to. This particular client has 2 separate companies but under same roof. They only differentiate the companies by location but using same inventory. 2 companies are Trading and Manufacturing. Manufacturing company will have 2 location. (1.RawMat and 2.FinishedGoods)

Ok, the situation is like this. When, there's a project from a customer, Trading company will buy raw material from a vendor and place it at Manufacturing company (Location=RawMat). At purchase order I already specify the Job and Job Task No for each line item. This particular items need to have Serial No as well. So, at Item card, I already setup the serial no for the items.

Manufacturing company will do assembly order for the item and then once they finished the assembly order, they will place it under location=FinishedGoods.

Now the issue comes in as below.
1. When I do purchase order and specify the job, and post receive, there's negative adjustment being done at the ledger for the item.
2. When, at assembly order, I am trying to create and assign serial no for the item, it does not letting me to do it. It says the Serial No Availability=No. Therefore, I can't complete the Sales Order to sell back to Trading company.

Anyone can help me to advise whether anything I am doing here is wrong? Or any work around to tackle this issue?