Learn English – What English idiom or proverb is equivalent to this Hindi expression: “Having eaten 900 rats, cat moves to Hajj”

Here's one that highlights the problem with relying only on the theoretical at the expense of the practical:

In theory there is no difference between theory and practice. But in practice there is. - reported by Walter J. Savitch, originator unknown but possibly Jan van de Snepscheut according to snopes.com

(Thanks to @DCShannon for the snopes link for attribution.)

John Ray, A Collection of English Proverbs (1678) offers an English saying that expresses the gist of the Hindi proverb you relate:

Steal the goose, and give the giblets in alms.

The underlying notion here is that a thief may take a thing of relatively great value for himself and then donate a pittance from his ill-gotten gains to the poor as proof of his charity or magnanimity. More broadly, the proverb criticizes individuals who take what isn't rightfully theirs, give a mite of it back, and then claim to be people of great philanthropy or piety.

Although I have never heard a person in real life repeat this proverb, it has appeared in many collections of English proverbs over the centuries, including The Macmillan Book of Proverbs, Maxims, and Famous Phrases (1948). Evidently, a saying from Spain uses a pig instead of a goose to make the same point. Henry Spence-Jones, The Pulpit Commentary, volume 4 (a nineteenth-century text), after repeating the goose-and-giblets proverb, mentions this Spanish alternative:

"Huerto el puerco, y darlos pies por Dios," "Steal the pig, and give away the pettitoes [trotters] for God's sake."