Bhai Manjh was a follower of Sakhi Sarwar, a Sufi pir with a very large following in Punjab, who came to Guru Arjan Dev Ji and asked to become a Sikh.
The account is that he was told to choose one or the other, gave up the Sarwar following, lost the standing and income that went with it, and was set to cutting and carrying firewood for the langar.
One night, carrying wood in the rain, he fell into a well. When help arrived he asked that the wood be lifted out first so that the langar's fire would not go out.
Guru Arjan Dev Ji is recorded as saying that Manjh was the Guru's beloved and the Guru was Manjh's.
The story is about the cost of changing allegiance in a village society where a following was also a livelihood, which is what the first paragraph is doing and is usually skipped over.
Sakhi Sarwar's following in Punjab was enormous into the twentieth century and has almost vanished, which is a religious change nobody has written about.
He came to Guru Arjan Dev Ji from a following of Sakhi Sarwar, which is the detail the story turns on: he gave up an established devotion for a new one.
The account is of him carrying firewood for the langar in the rain, falling into a well with the load, and refusing to let go of it when he was pulled out.
The phrase attached to him in the tradition is that he was the Guru's beloved. The story is about the cost of the choice rather than about the reward for it.