Как по Английски будет на безвозмездной основе

idiom

Done for the public good without compensation

adverb

(especially with reference to legal work for a client with a low income) without charge.

Примеры использования:

The attorneys are representing him pro bono

adjective

Denoting work undertaken without charge, especially legal work for a client with a low income.

Примеры использования:

Pro bono legal services