Beschlossene Sache — a decided matter. It is no longer open.
Unter Dach und Fach — under roof and in its compartment, that is, safely inside the building. The work is finished.
In trockenen Tüchern — in dry cloths. The matter is safely concluded.
Festschreiben — to write something down so that it is fixed.
Nägel mit Köpfen machen — to make nails with heads. Do it properly, so that it holds.
Der Drops ist gelutscht — the sweet has been sucked. It is over and decided.
Ein Paket wieder aufschnüren — to untie a package again. In politics, it means reopening an agreed compromise, and it is almost always said as a warning.
Rolle rückwärts — a backward roll. The press uses it, usually with mockery, for a politician who reverses a decision.