Use a classic sentence or poem to express that you will keep your promise.

It's a deal. A word spoken is past recalling. A promise is a promise.

A word spoken is a word that must be honored.

A golden mouth and a jade word.

It's a deal.

Breaking a promise is failure.

Mozi's "what you say is what you do" and Soong Ching Ling's "what you say is what you believe, and what you do is what you do"

Keeping a promise is like defending your honor. -Balzac

If you slip, you can stand up at once; If you break your promise, you may never get it back. -(America) Franklin

Treat people with sincerity, and people will not bully me; Nothing can be done by being honest about things.

—— (Middle) Feng Yuxiang

Credit is both an intangible force and an intangible wealth.

-(Japanese) Kōnosuke Matsushita

It is more important for a person to keep his promise than to protect his property. -Moliere (France)

Credit is like a mirror, as long as there are cracks, it can't be connected as before. -Amir

All that glitters is not gold; Beautiful words are not all good words.

-(British) Shakespeare

Keeping a promise is better than being famous. -(America) Roosevelt