A shagun is a token given to mark an auspicious occasion: money, always in an amount ending in one, a rupee added to a hundred so that it cannot be divided evenly and the relationship continues.
The one-rupee coin glued to an envelope is universal across north India and the logic behind it is specific and is rarely explained.
Shagun is given at engagements, weddings, births, house openings, first earnings and departures. It is recorded, and the record is kept, because it must be reciprocated at an equivalent or greater value on the corresponding occasion in the giver's family.
That ledger is a real financial instrument. A family marrying a daughter calls in what it has given over twenty years.
Wedding gift registers are kept in notebooks and consulted, and the obligation passes down.
It is a mutual insurance scheme operating across a village and a kin network with a memory of decades and no written contract other than the notebook.
The shagun is the auspicious gift and the sagan is the occasion of giving it: a betrothal, a wedding, a birth, a new house, a departure abroad.
The amount is recorded. A book is kept at weddings listing who gave what, and the return at the next family's occasion is calibrated against it, which makes the whole thing a long running mutual credit arrangement.
The rupee ending in one is the standard form. The extra rupee is what makes the sum incomplete, and an incomplete sum implies continuation.