Regards renting the car. The expense comes if you want to rent it in one country and return it in another. So you could either rent it at Frankfurt Airport and return it there, or get a train from Frankfurt to somewhere in France and rent and return it in France.
Where were you planning to return the car?
You have picked some obscure places. I found Vahl-Ebersing (postcode 57660) south of St-Avold
But no Fremstroff, just Freistroff (57320) and Frémestroff (57660). Frémestroff is next to Vahl-Ebersing, so I assume you mean that.
St Avold has a station. There is a train every hour from Frankfurt airport to St Avold, journey time 3 hours, change at Mannheim and Saarbrücken. See www.bahn.de for details. I have no idea what if any car rental companies exist there.
Strasbourg to Vahl Ebersing is 1½ hours by road (see http://www.viamichelin.com ), so your best option may be to take a train to Strasbourg and hire and return the car there,