What should a Muslim living in Germany do with 20 euros found next to a metro station, knowing that the minimum an individual there receives as government aid for the unemployed is 400 euros in cash, 400 euros for housing, and health insurance?

The money you find whose owner you do not know is a lost item (Luqatah). If this money has no value and its owner would not pursue it, then it is not obligatory to announce it, and it is permissible to benefit from it. Imam Ahmad and some scholars were of the view that the estimation of a trivial item refers to common practice ('urf). So, if what you found is insignificant and its owner would not search for it due to its triviality, then you may benefit from it without announcing it.