Why would they sell? The Congo was profitable, although keeping it so required consistently criminal policies, to a point that even Belle Epoque respectable society Europeans bothered to show disgust.
And why would Russia even consider to buy? They have no strategic interest in the area nor the logistic capability to project power there.
Its not as hard as you might think. It would really only require two things: (1) Belgium's position is rendered more unstable and (2) Russia buys into the idea that the mark of a Great Power is territory in Africa.
The idea of purchasing a preestablished colony from a mid-sized state that is down on its luck might be a lot easier to justify as Russia may be able to leave much of the colonial infrastructure in place, while at the same time reaping its rewards. Granted, it would make more sense for them to establish coaling stations in East Africa - but should England preempt this move by taking them for herself, or encouraging a faster allocation of those territories to non-Russian aligned states, then Russia would naturally be forced to look elsewhere should they be sufficiently invested in the idea of African Colonialism. As for Logistics, you shouldn't assume autarky. Russia would have been more than capable of establishing relationships with other states in order to enhance its own power-projection capabilities. Belgium to the Congo is not all that closer than, say, the Baltics or the Crimea to the Congo.
As for France, if France's involvement in the first prerequisite condition is sufficiently large and negative, Belgium might be able to use that as a pretext to ignore it.