promissory

As a letter s

is the letter s with a.

As an adjective promissory is

containing or consisting of a promise.

As adjectives the difference between promissory and promising

is that promissory is containing or consisting of a promise while promising is showing promise, and likely to develop in a desirable fashion.

As a verb promising is

present participle of lang=en.

As a noun promising is

the act of making a promise.