@skaufmann , assuming that you are a US parson ( citizen/ GreenCard ) from 2012 till date i.e. US laws were and are applicable to you for the duration:
(a) Germany , while you were/are a resident, can indeed tax ypou on the disposal of such assets and execute such based on its laws, irrespective of US tax laws. ( if I remeber correctly Germany taxes you on world income while being a resident of Germany ). Thus Germany will indeed com pute the gain based on Euro of the day ( both for acquisition and disposal)
(b) For US tax purposes, US$ being operative currency all transactions are ij US$ of the day ( or average of the year ).
(c) Since both jurisdictions are taxing the same income you are eligible for foreign tax credit ( and thereby ameliorating the double taxation bite ) using form 1116. Note however that while the US will recognize the total foreign taxes paid, the amount allowable in the current year is based on a ratio of Foreign income to world income ( and thus is never 100% ). You can also take the foreign tax as a deduction ( but here you have to contend with the SALT limitations ).
Does this make sense ? Is there more I can do for you ?
pk
