Dictionary:

inconvertible

  (ĭn'kən-vûr'tə-bəl)

1. Not redeemable for money in coin: inconvertible paper currency.
2. Not able to be legally exchanged for another currency.

Currency or security that cannot be freely exchanged for another, or sold for cash. An inconvertible currency is less desirable to investors than a freely Convertible currency.

The adjective has 2 meanings:

Meaning #1: used especially of currencies; incapable of being exchanged for or replaced by another currency of equal value
  Synonyms: unconvertible, unexchangeable
  Antonym: convertible (meaning #1)

Meaning #2: not capable of being changed into something else
  Synonym: untransmutable