Well, the real problem is that you want to leave a chance for the underdog to win, but have to avoid rewarding a player for being last. What you
don't want is to negate the possibility of a player gaining an advantage; after all, that's what both sides are supposed to be working towards. It sucks to be winning every battle and then have those victories count for nothing just because the map is triggered to help the loser; winning, not losing, should be rewarded. So increasing the income of the losing player isn't really a good solution. If anything, a one-shot income boost would be slightly better, because it lets them start pushing back at the winner, but the using-up of the emergency money counts as an accomplishment for the winning side, who will still have their own emergency money coming to them if they get pushed back that far. Sure, it might be a less "professional" solution, but professionalism for its own sake is worthless.
Building in a turning point at the middle can be a bit better, but all it really does is slightly delay the onset of the original problem while simultaneously adding an incentive for a team to deliberately give ground at the beginning. The more you stretch out the battle to "tilt the balance", as it were, the more unsatisfying it will be for the players to do anything, because they'll get so little progress towards that tilting point back in return for their effort.
The only truly adequate solution to the problem is to make strategy sufficiently important to the gameplay that a weaker side can defeat a stronger one through superior play, be it by outmaneuvering the opponent, choosing a different form of attack, or some other means. Ideally, a game has one or both of the following two possibilities:
1) The weaker player can, without any artificial "help", become the stronger. This might happen through being able to rebuild in secret in another part of the map, being able to bypass enemy forces to take over territory, being able to develop a counter to the opponent's forces, etc.
2) A weaker player can win without having to take over as the stronger. Perhaps there's some way to pull off a decisive attack on a certain target to end the game, something that a fast, heavily-armed strike force might accomplish if skillfully maneuvered through the opponent's defenses, or some way to kill the enemy by landing a good shot with some weapon or spell even though they are the stronger one.
Probably the best example that I've seen of this sort of balance is BattleCruiser Command. Builder losing ground? Relocate and rebuild; get some defenses up in a new place before they know you're there. Enemy troops mopping up your bases? Have your builder get up some air before dying and make a strike at the other cruiser. Can't hit the other ship hard enough? Board it and kill it from the inside. There's almost always hope, even if it comes down to trying to get the other ship to overextend itself as it kills the last of your base, but at the same time, any advantage is a real, material advantage, not something trivial that the other team can take away because the map is designed to prevent advantages. That balance might have something to do with why that map is still played after so many years despite needing a decent group of non-newbs to get a game going.
The point of all this? There's no decent solution that you can just tack on to a map to fix the advantage problem; the gameplay has to be designed from the start with being, well, a good game in mind. Yes, that means you can't take any old idea, no matter how "cool" it might seem, and make a good game out of it; sorry, but too bad.
None.