From Wayback machine - in September 2018 the GTC stated this:

"If you fail to return the equipment, cables and accessories or return damaged equipment, cables and accessories you must pay the compensation in accordance with the product information on fiber.salt.ch."

Unfortunately the full product and tariff brochure from that time isn't archived, but the product FAQ is and it says:

Salt Fiber Box:
The Salt Fiber Box (device and accessories) remains the property of Salt during the whole contract duration.
At the end of the contract, you have to return the whole equipment (including the original packaging) undamaged to Salt within 30 days.
If you fail to return the material - or if you return it damaged -, a compensation fee of maximum CHF 199.- will be charged (details ref. Product & tariffs brochure / Return of equipment).

So while you may have a point morally and from a CX viewpoint, contractually you seem to be completely wrong.