My response is going to seem critical, but don't let that dissuade you from thinking up improvements for the game and hey mine is just one opinion so you may feel that my input doesn't have merit anyway.
I think that right off the bat, any improvement idea for the game, especially a 'why can't we do _____' is not to think about your idea in how you would use it, but how it could be exploited. Anything that involves one party giving something to another party without getting anything in return should rightly be met with plenty of scrutiny in this game. Think of how a multi would exploit it. The most significant way to prevent multis is to make it not worth it.
With no restrictions for joining federations currently it would be effortless to just create a new account, sell off the new country's assets, join the federation, and give all of the money to the main account. That could easily be 5T or more for a couple clicks. Too easy to exploit. All of the best intentions will still hurt the health of the game if the improvement makes it easy to cheat.
Exploitation aside, the concept still doesn't feel right. A federation is a military alliance. Its core concept is you protect me militarily and I protect you militarily. In conflict crises when the enemy is bearing down on your friend you don't send him lunch money, you send him a gun.
Now turning your concept towards the common market could be interesting. The common market has plenty of flaws that need improvement, one of those concepts is promoting an interactivity that sees multiple players working together rather than the current system used by many where their common market is just their empire's countries or no common market at all. I still thing just giving money is too much of a trouble situation, but you could promote adding a feature to common markets where members can offer or request loans from one another with higher limits than the current world loan system and perhaps even at a slightly lower interest rate. That way say your federation allies are also in a common market with one another, one that is in conflict could have instant access to funds but in loan form rather than gifting.