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I forgot to collect my money on the correct day. The banker conveniently forgot all of it. I can give him new money now, but the old one seems to be gone.

Maybe it'll turn up again if I give him more?
Ever dealt with a real investment banker? Yup, same here: Your money is gone, and you have no legal way of murdering the chap and turning his remains into a fancy throw rug.

Keep away from the banker. IIRC there's an achievement on Steam associated with him, but since we lack those, you might as well save the 70.000 gold required to get him in the first place.

No idea if the devs are going to rework him at some point, the return of investment is ridiculously low, and losing all the invested money if you don't pick it up before the 7 days are over is a noob trap.