Word request: "proprietary currency" or "artificial money"

A term used historically is scrip:

[A] term for any substitute for legal tender and is often a form of credit.

Scrips were created as company payment of employees under the truck system and also as a means of local commerce in times where regular currency is unavailable, such as remote coal towns, military bases, ships on long voyages, or occupied countries in war time.

Note that this might have negative connotations, since scrips were used rather exploitatively in many places as a means for employers to control their workers and increase their dependency on them.

A more neutral and universal term would be local currency:

[C]urrency not backed by a national government (and not necessarily legal tender), and intended to trade only in a particular area.

The word token would also be appropriate.

The key point of difference between a token and a coin is that a coin is issued by a governmental local or national authority and is freely exchangeable for goods or other coins, whereas a token has a much more limited use and is often (but not always) issued by a private company, group, association or individual.

token (BrE)/ (noun)

A piece of paper that you pay for and that somebody can exchange for something in a shop/store.

• Collect six tokens for a free T-shirt.

  • a £20 book/record/gift token (oxfordlearnersdictionaries)

coupon/voucher (noun)

A voucher is a certificate or document that either allows you to purchase something or proves that you paid for something

• The team also gives them vouchers for meals at sponsor restaurants. (Washington Times May 29, 2015)
  • The vouchers are redeemable at a range of stores. (Washington Post May 27, 2015) (Vocabulary.com)