example.move

module p::example {
  use sui::coin::{Self, Coin};
  use sui::dynamic_field as field;
  use sui::object::UID;
  use sui::tx_context::TxContext;

  struct Manager has key { id: UID, /* ... */ }
  struct Label<phantom COIN> has copy, drop, store {}

  entry fun handle_coin<COIN>(m: &mut Manager, coin: Coin<COIN>, ctx: &mut TxContext) {
    let l = Label<COIN> {};
    if (!field::exists_with_type<Label<COIN>, Coin<COIN>>(&m.id, l)) {
      field::add(&mut m.id, l, coin::zero<Coin>());
    }
    
    let manager_coin = field::borrow_mut<Label<COIN>, Coin<COIN>>(&mut m.id, l);
    // Do what you like with manager_coin and coin. 
  }
}