Return the lock.amount after fees are taken.

    Fees are taken only for the outgoing channel, which is the one with
    collateral locked from this node.

def get_lock_amount_after_fees(
        lock: HashTimeLockState,
        payee_channel: NettingChannelState,
) -> PaymentWithFeeAmount:
    """
    Return the lock.amount after fees are taken.

    Fees are taken only for the outgoing channel, which is the one with
    collateral locked from this node.
    """
    return PaymentWithFeeAmount(lock.amount - payee_channel.mediation_fee)