Personally i would look to sell him with a big sell on clause inserted.
We took a huge gamble when we signed him, he had only made about 15 starts i think at the time and we spent £12-£15m on him (i think £3m was clauses which i cant see we would had to of paid as we nor him have done anything since he joined) we basically brought potential. The germans are snapping up british talent at the moment and they gave up on him after less than a year and probably couldnt believe their luck when we made an offer.
We probably thought if it comes off then we could double our money on him being a young british talent,
Instead the potential is nowhere near being fulfilled and hasnt looked even remotely close to doing so, he has done nothing of note for us, he hasnt had a load of chances to be fair but even when he has there hasnt been a moment where you have thought 'ah, there is that potential'
Even this season with us dropping down a league he hasnt got in, he has now gone to play for the best club in scotland, i think if you put HRK up there he would get 30 goals a season, thats the standard.
It will probably do Burkes confidence good, If he is happy to stay there then i think the maximum we would get outright would be between £3m - £6m (depending how he does between now and the end of the season), i would take that and insert a big sell on clause just incase he does prove to be a star, however my gut instinct is the £3m - £6m is what we would end up with and just have to accept it was a gamble that backfired.
I hope i am wrong and he either comes back and is brilliant or goes on to be a star elsewhere, but i think he will still come unstuck when he faces better opposition if / when he moves on from scotland, quite simply because i dont think he is a particularly good footballer or has a football brain, i think he is a athlete who will be effective in poorer leagues, better players will suss him out.