Hello, I am a solicitor with 20 years experience. I will try to help you with this.
Who was it said that you could keep it and is there a record of it in writing?
OK I am not sure this is entirely straightforward. If your line manger didn't follow the right procedure it is easy to see why theu want it back. Did the line manager have authority to gift you the ipad?
If he had the authority to give it to you an gave it to you then yo are able to keep it in law. However if it comes to a dispute it might be hard to prove. The leaving inventory would be a document that might support your case. If they say were misled by the line manager that is not your problem. Indeed by saying that they may well be admitting that it was given to you by your superior at work. If they have accepted in writing that your line manager did give it to you then you might want to suggest that their real dispute is with the line manager not you and you are entitled to accept a gift given by someone who as far as you were aware was able to make such a gift..
It really depends on what you want. You have a defendable position here and they might not really go to court to get it back if you called their bluff and even if they did you have a defence which might well work. However, is there any chance you may need a reference from this company in the future? If so you might have pragmatic reasons for giving it back.
The invoice wouldn't prove anything in this case.
You could do that and call their bluff. If they do go to court it would be in the small claims court and they would not be able to get any legal costs paid by you. They could also not get an order for return of the ipad only money to compensate them for the loss. It may be that they don't go to court but you could always wait and see and if they do then return the ipad to them to avoid having to contest a court case and run the risk of an adverse finding.
It depends you have a defence but if calling their bluff doesn't work yo will need to decide whether it is worth fighting and yes returning the ipad would be a way of settling this case.