bmi slots and routes
bmi no longer flies to Bru or Ams - along with Paris it gave up these short European routes some time ago, partly I recall due to the Channel Tunnel and partly to use the slots for more profitable routes. As I have reported before, it also gave up Fra in favour of Lufthansa and, I think I recall, also gave up some Scandanavian routes to SAS - all part of the Star arrangements.
Buying, selling, temporarily renting out or leasing in LHR slots is common. I recall Virgin, bmi, I think some of the US airlines and Aer Lingus have all had a go at it at one time or another. For example, when Virgin started to serve India, it did so with slots "borrowed " but unused by Air India. Luxair also sold its LHR slots some years ago when it moved to London City and to raise cash. An airline leaving LHR in favour of LGW or Stansted is often a sign that it needed the cash.
Mergers and partnerships can lead the "authorities" to insist that slots be surrendered in the interests of competition - thats why the first AA/BA partnership proposal fell - the authorities demands to give up slots were considered excessive by the 2 airlines.
In these circs the authorities then reallocated based, I think, on underserved routes, what airlines have "bid for" and so on.