Edit: money supply and currency supply are two separate things. Money is the abstraction of anything of value. Currency is something issued by a currency provider/central bank.
So how is money supply constraint eliminated by Promise Language? By making any store of value available to facilitate the transaction. The simplicity is so pure, every currency on earth would become a store of value.
Currencies have two aspects:
1. store of value (wealth storage)
2. medium of exchange (wealth translation)
Electronic transactions allow near instant exchange (translation) of value.
It is possible for a pure "medium of exchange" currency to be created within this system. Expands and contracts with the volume of transactions, however, that is outside the scope of current technology. Promise Language would have to be implemented prior to that kind liquidity to occur.
If the Federal Reserve implemented Promise Language, the Fed would suddenly have so much liquidity with other forms of value that contrary to most economic theories, the good money would chase out the bad. Of course, existing contracts denominated in dollars would have a real effect, the underlying tide would tend towards wiping out dollars to their intrinsic value. IE. dollars would become scarce as other alternative value translations became available. EG. Bitcoin or electronic gold or silver.
NOTE: the above applies to the derivatives world. Real world contracts/salaries denominated in dollars would become more valuable (increase in purchasing power) by contracting the mess at the "financial" level.