Let's (from an ousiders view) break this down:
1 - Barker was loaned to Berwick, until he got injured then Berwick took him out of the 1 - 7, so, when he signed for Peterborough, he wasnt on loan at the Bandits and therefore entitled to sign for the Panthers (or anybody else, in fact).
2 - Who "dropped" Ben barker has nothing to do with it. the Worker thing is a red herring on your part. I'd have a sick-note saying I'm off for a month (or whatever) but If I sign back off the sick before then, I can still go and work for who I like - depending on the contract. Barker isn';t employed by Berwick so is under no pressure to go back there at all.
3 - Barker may or not have misled Berwick. He was told his season was prob over and reported such to Bandits. Medical opinion can be often wrong and this one of those cases it seems to me.
Nothing smells iffy.