Rent sleeping bag?

The sac à viande is actually a sleeping bag liner, not the sleeping bag itself. These can be purchased fairly inexpensively at outdoor stores such as REI. An outdoor store in Chamonix may also have them, but they may be cheaper here.

I think the Refuge is referring to a "sleep sacks" or travel sheets. They are similar to a bed sheet but made into a sack or bag, typically rectangular for hostels and mountain huts. The use to require them for hostels and mountain huts that provide duvets but don't have the capacity (or interest) in washing the duvet covers with a change in guest. The cheaper standard material is cotton but for lighter weight (ski touring, trekking etc) silk sleep sacks are nice.

https://www.cocoon.at/products/index.php/p/travelsheet_mummyliners_en

Thank you all so much for this helpful advice on these sleeping sheets/sleeping bag liners. I also discovered that this refuge has disposable ones, but I wonder whether they are comfortable. I will probably buy one online to take.