Mongo DB remove 1 item from an array

I have a buy system in place for my discord bot where it adds something to the inventory array in mongoDB:

data.Inventory[itemTobuy] ++;

It gets stored like this:

pizza: 1

And i have a use system where you can use items in your inventory. I would like to remove the whole thing if theres only 1 instance of this item but if there is more i would like to decrement the value by one. Say from pizza: 5 to pizza: 4.
Althought im only 25% sure on how i would do such things

The full code for the command that adds the item if needed:

const { Client, Message, MessageEmbed } = require('discord.js');
const { User } = require("../../databasing/schemas/User")
const inventory = require('../../databasing/schemas/inventory')
const items = require('../../items/shopItems')

module.exports = {
    name: 'buy',
    /** 
     * @param {Client} client 
     * @param {Message} message 
     * @param {String[]} args 
     */
    run: async(client, message, args) => {
        userData = await User.findOne({ id: message.author.id }) || new User({ id: message.author.id })
        const itemTobuy = args[0].toLowerCase()
        embed = new MessageEmbed({ color: "#2F3136" })
        if(!args[0]) return message.channel.send({
            embeds: [ embed.setTitle('<:redCross:1004290018724020224> | Please mention something to buy!')]
        });
        const itemid = !!items.find((val) => val.item.toLowerCase() === itemTobuy);
        if(!itemid) return message.channel.send({
            embeds: [ embed.setTitle(`<:redCross:1004290018724020224> | ${itemTobuy} is not a valid item!`)]
        });
        itemPrice = items.find((val) => val.item.toLowerCase() === itemTobuy).price;
        userBalance = userData.wallet;
        if(userBalance < itemPrice) return message.channel.send({
            embeds: [embed.setTitle(`<:redCross:1004290018724020224> | You dont have enough money to buy this item(LOL)`)]
        }); 
        const param ={
            User: message.author.id
        }
        inventory.findOne(param, async(err, data) => {
            if(data){
                const hasItem = Object.keys(data.Inventory).includes(itemTobuy);
                if(!hasItem){
                    data.Inventory[itemTobuy] = 1;
                } else {
                    data.Inventory[itemTobuy] ++;
                }
                await inventory.findOneAndUpdate(param, data);
                } else {
                new inventory({
                    User: message.author.id,
                    Inventory: {
                        [itemTobuy]: 1,
                    }
                }).save()
                }
                message.channel.send({
                    embeds: [embed.setTitle(`<:greenTick:1004290019927785472> | Successfully bought ${itemTobuy}!`)]
                });
                userData.wallet -= itemPrice;
                userData.save()
        })
    }
}