Imagine i buy something today with btc.
With 1 btc.
Today its worthed 60k tomorrow will be 40k.
And can fall down even further so the person who i transfered btc are in loss ?
The volatility is main issue why we cant use never for paying something....or im wrong ?
Or how is the situation here ?

It also depends on how the seller is looking at it.
If I have something that cost $45000 and sell it to you for 1BTC which as of now is $51000 I can convert most of it to fiat / USD and sit on the rest as an investment.
If it goes up I make even more. If it goes down I make less. No matter what I have converted enough to cover CoGS so it's not a big deal.

It's just like taking any non local fiat for something. If between the time I sell something and take a payment in other then $ (I am US based) I can take a hit. EURO vs USD changes and I sold something to someone in Germany I could take a big hit by the time I get my money.

-Dave

The seller can decide to convert the $60 000 on the same date of the transaction if he/she needs the Fiat currency .... or they can hoard it and wait for the price to reach say... $65 000. So you can win and lose... depending on how you want to play it. 

A lot of merchants make use of Payment processors... and they can tell the payment processor to convert say 80% back t Fiat currencies and to hoard %20 until the price hit a specific price and to decide to sell then. 

^ When it is used to make payment there is no risk. Suppose you hire someone to do a specific job, if his monthly salary is $10k worth of BTC then you have to pay exactly $10k worth of BTC, and the employee gets exactly $10k worth of BTC, there is no loss to any party.

Now let's talk about volatility, don't think of volatility as a problem, look at it as a blessing. If the price of BTC goes down from $60k to $40k then it is a new buying opportunity. Stop selling, keep buying the dip. whatever you have in your holding you buy more at a low price. If the price goes up then sell, there is nothing to lose here.

Imagine I buy something today with U.S. dollars.
With enough U.S. dollars that the seller can buy 1 Big Mac from McDonald's for himself.
Today it's worth enough to buy 1 Big Mac, but because of inflation McDonald's changes their price and tomorrow it is not enough to buy a Big Mac anymore.
And because of inflation, they can buy even less for it later in the future, so the person that I paid with U.S. dollars in in a loss?
Inflation is the main issue why we can't ever use U.S. dollars to pay for something... or I'm wrong?
Or how is the situation here?

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With Bitcoin, the seller has options.
If there is a different currency that they trust more, then they can exchange their bitcoins for that other currency immediately after the sale.
If the seller doesn't need access to the funds immediately, then they can just wait for the exchange rate to recover back to what it was.
If the seller has things that they need to purchase, then they can make that purchase immediately after the sale (before the exchange rate changes much).

So far I have not been able to match the value of Bitcoin with certain items, even items that are considered luxury. Be it a car or a house. But if I choose, of course I will use Bitcoin to pay for home facilities that are beneficial for me and my family. The rest I have not been able to balance with other luxury items. That is when viewed from the current value of 1 Bitcoin. Maybe with Dollars I can relax a lot more than Bitcoin.

I don't think you understand what a stock market is. Someone's profit is definitely going to be someone's loss eventually, but this thing won't hold true until the market completely collapses to zero. Because someone to whom you are selling in profit is buying for anticipation that in future the prices will eventually rise, and similarly when he would sell, he would sell to someone who thinks price would eventually rise, so until the price is rising everyone is in profit, some might be in little profit while some might be in more, but once the market starts collapsing then this chain gets reversed and it becomes more like a zero-sum game. If the 3rd person in the chain doesn't sells at right time he is eventually taking the loss from the profit that the first two guys have made.

OP is just giving the example of what if he buy something from using Bitcoin, let's say 1 BTC and the current price is like $60k and after the transaction with the merchant it goes $40k or something like that. I think we all know how volatile the market is, so, I think the merchant wouldn't accept something if he knew the market is in a decline or it's just me thinking that way?

Besides, if the market is in decline there's no way the price would drop straight from $60k to $40k in a minute, it should take some time though I get the point of OP and it's just a what if. So, it's really up to the merchant to decide the price but the question is that is it fiat based or Bitcoin based? Like you have to pay $60k for that item no matter how much Bitcoin it is or you pay 1 BTC no matter what's the value of it on fiat.

that's why Bitcoin is difficult to use as a means of payment. The price of bitcoin always goes up and down so that Bitcoin holders can benefit or will be harmed when the price goes down. If they can manage the bitcoins they have then they can still make a profit, the most important thing is that they can hold Bitcoin when the price goes down because one day the price of bitcoin will definitely go up more.

i bought something with fiat in a retail store that only accepts fiat.

its regular price was $400. but was $300 on black friday. and now that christmas is approaching and stock is low its priced at $450

even fiat has volatility.

ive even seen simple things like milk vary by 25% depending on which retailer you choose.
fiat is not as stable as people think. we have just got used to its change so much we have stopped noticing the change.

i have just gone on ebay and typed in "UK XBOX X series 1TB" and the prices range from £700-£760
so fiat is no different to bitcoin in terms of variable pricing

when you put the same mindset of "sales event", "discount day", "convenience premium" and £retailer competition/monopoly" of fiat, and put it to bitcoin.
you will start to see a bitcoin price dip as a discount day (good for buyer bad for seller)
and when the price is hyped up its the "convenience premium" (good for seller bad for buyer)

so buy the dip(discount) sell the hype(premium)
buy low sell high

when transacting. merchants decide their level of risk of discount/premium when they set their prices to offer products for. they assume the risk. they assume they may make a loss if they dont time their sell of goods right (much like retailers make a loss on goods passed their 'sale by date') or when next gen fresher goods come about

they price this risk into the goods offering price.

if they want to sell something at ~$500(0.01btc today) but they think that by the time they get that btc and convert it to pay their own costs the price might dip by 10%. they might price it at 0.011btc instead to factor in this risk. ensuring it covers any possible dips.

many merchants even have 'live price' monitoring, where by the btc price of a website shopping cart calculates the amount of btc only at the time of purchase or uses dynamic webpages where the day/hour the customer is browsing has the price of goods match that day/hours price. and not some fixed html text they only update every 6 months (dynamic websites are a thing)