But, the deeper we get into this, the less secure I feel about having money for free (wintertime) - as this wouldn't add value to the game.
One possible value that could be added to the game is the temptation to spend the money now vs saving it to earn interest. It would be a balancing act to find a point where the player feels the need to buy stuff with the principle instead of waiting for interest to accumulate. If interest gathers very slowly, the player is likely to just say 'screw it, I don't wanna wait.' but if the interest is a significant increase, then the player may be tempted to hold off on buying something with the principal, and use the interest instead.
Sort of related is that infinite/renewable resources are fairly common; mob loot drops are a continuous stream of unending stuff, tree logging, farm growing, animal breeding. So if the game employs any of these things, and they are convertable to money, then there is an infinite supply of money.
Civilization V has a process that seems similar to this: The player can put a citizen onto a forest hex and receive an infinite stream of logs, or they can chop down the forest and recieve a single lump sum of logs.