In accounting terms, you might consider the stake to have been bought for 25 billion at the time, but now it's being considered worthless (or maybe even a liability) by BP, so something of no (dollar) value is being given to Russia. The loss in value might be considered to have occurred as a result of recent events.

Edit: note that I have no idea of the value myself. I'm just saying that if you were to accept BP's view, then they're not necessarily giving Russia a gift of any kind.