Update one receipt item

What your assistant is told

Correct ONE line on a receipt — its name, its price, its quantity, or (most often) the category it counts in. itemId is the item's UUID from get_receipt or set_receipt. This is a SPARSE patch: pass only what changes and every omitted field keeps its value; pass an empty string for quantity to clear it. Recategorising a line is the common edit and is why this tool exists — use it instead of resending the whole receipt with set_receipt, which would replace every item. A price that would push the items above what was actually paid is refused (items_exceed_payment); fix the transaction's amount first if the receipt is right. Returns the whole receipt as it now stands, including the per-category subtotals and the "Not itemised" remainder, so you can tell the user what changed.