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Safe place for 50k because of debts

Also you could rent one of these self storage boxes, put a bunch of junk there and one safe deposit box with cash in it.
I guess this is way cheaper than safe deposit box in bank, it's not reported and nobody will suspect you hold cash there (security by obsecurity).

He might as well rent out a Post Office Box at local post office then. That would also be cheap. Just make sure its one of the deep boxes or ones with flap where people can't use fingers in and grab out letters.
 
I am on the road all the time and don't stay in the Netherlands.
I was just afraid that German customs make random checks on the border as this is what I red on their website. I know that they make random checks on the German - Dutch border, they have stopped my bus (Flixbus) regularly and checked ID documents and occasionally also luggage.
Not only at the border, ZOLL can check you anywhere in Germany on the road.