But it might be best to use the Latin negotiorum gestio and place spontaneous agency in parentheses.

Die Geschäftsführung ohne Auftrag (lat. negotiorum gestio) ist die Wahrnehmung von Geschäften des Geschäftsherren (Negotiorum dominus) durch den Geschäftsführer (Negotiorum gestor), ohne dass der Geschäftsführer durch den Geschäftsherren hierzu beauftragt oder anderweitig in besonderer Weise ermächtigt wurde.
http://de.wikipedia.org/wiki/Geschäftsführung_ohne_Auftrag

Negotiorum gestio (Latin for "management of business") is a form of spontaneous voluntary agency in which an intervenor or intermeddler, the gestor, acts on behalf and for the benefit of a principal (dominus negotii), but without the latter's prior consent. The gestor is only entitled to reimbursement for expenses and not to remuneration, the underlying principle being that negotiorum gestio is intended as an act of generosity and friendship and not to allow the gestor to profit from his intermeddling. This form of intervention is classified as a quasi-contract and found in civil-law jurisdictions and in mixed systems (e.g. Scots, South African, and Philippine laws).
http://en.wikipedia.org/wiki/Negotiorum_gestio

Law Dictionary: What is NEGOTIORUM GESTIO? definition of NEGOTIORUM GESTIO (Black's Law Dictionary)
Lat. In the civil law. Literally, a doing of business or businesses. A species of spontaneous agency, or an interference by one in the affairs of another, in his absence, from benevolence or friendship, and without authority.
http://thelawdictionary.org/negotiorum-gestio/