At work I sometimes cut a corner when I know no manager is watching. If the result looks fine and no one is harmed, why does it matter?

Inside the office, your description may well be accurate. No manager is watching, the corner gets cut, nobody notices. But work does not happen in a sealed room. Whoever has gone home and whatever the hour, one gaze remains, and the Prophet (peace be upon him) put the whole matter in a single sentence.

“Ihsan is to worship Allah as though you see Him, for though you see Him not, He surely sees you.”

The Prophet (peace be upon him), Sahih al-Bukhari and Sahih Muslim (Bukhari 50 / Muslim 8a) · meaning

Notice what this does to your question. The cut corner was never really about the manager. It is a kind of forgetting, easy to slip into when the corridor is empty, and the reminder is simply that the corridor was never empty.

“Does he not know that Allah sees?”

Qur’an, Al-Alaq 96:14 · meaning

That is a waking call, not a scolding. And the same reminder comes elsewhere as company for the long day rather than as a threat.

“Indeed Allah is ever, over you, a Watcher.”

Qur’an, An-Nisa 4:1 · meaning

There is a well known finding that people behave more honestly when they merely feel observed, and that even a picture of eyes on the wall is enough to shift their conduct. The believer needs no poster. He carries the awareness with him, and that changes what the uncut corner is worth. Work no one will ever inspect stops being performance and becomes something offered. As for the result looking fine either way, it may. The difference is in what you become while working, and in whether the quiet satisfaction of a job done whole is yours at the end of the day. Habits also do not stay in one room; the hand that trims where it is safe will trim one day where it is not.

One small step: Pick one task tomorrow that no one will ever check, and finish it properly and completely, saying quietly in your heart, “This one is for the One who sees.”