Function delta 

pub fn delta(
    owner: SuiAddress,
    coin_type: TypeTag,
    coin: i128,
    address: i128,
) -> (Key, Value)

Build a (Key, Value) pair representing both sides of the balance change for (owner, coin_type) in a single merge operand. Either field may be zero; the merge operator’s field-wise sum makes a zero contribution a no-op against the accumulator.