This is an interesting situation.
First dealing with a potential buy out. I'd have to pose the question as to whether even if you did buy it out you would be allowed to rent? Isnt the whole premise behind the affordable housing scheme that the houses be for persons in need of housing and normally they contain conditions that dont allow them to be bought as investment properties.
Getting back to the buy out, you can't now buy it because the open market value is less than the councils value. Firstly did you have your own solicitor when entering into this scheme? If so, make an appointment and see if your legal documents covered this scenario. If you did not have your own solicitor then maybe ask the council in writing whether there is any clause that covers the situation where the value set by the council is higher than the current market value. You've been in this house for one or two years now ( it seems) but the fact that you have to move away for employment reasons means that the council might be disposed to be helpful to you. Have you contacted a local councillor or arranged an appointment to meet one of the housing officers?
I'd be surprised if the council wouldn't help in some way but I'd be interested to know what efforts you have made to date yourself. At the end of the day if all else fails there might be a test case in it.