i don't understand how does it works futures on BitMex. For example, i agrees to buy BTC at say $15k in one months time. Someone have to pay me in btc in which way? for example if the price is higher than 15k i will receive less than 1btc?
This is how I understand it.
So as we know futures contract is a contract for buying an asset (BTC in our case) for predetermined price. The buyer is
buying a contract for buying BTC at a price of 15k in one month (he already payed for it right now) and a seller is selling this contract (at this particular moment they are trading the agreement). Buyer can resell this contract or just drop it if the price will be much lower. In this case the seller will get a profit from selling the contract. But the seller can't refuse to sell the asset on the aggreed price. That may sound rough so I'll try to bring an example.
John and Andy are making a futures contract. John pays Andy 1k USD for the right to buy his BTC a month later for the price of 15k USD. So here are 2 possible endings:
1. In the end of the month Bitcoin worth 14k USD. This price is unaccaptable for John and he is giving up. He is disappointed because he lost 1k on buying this contract.
2. In the end of the month Bitcoin worth 20k USD. Andy have to sell BTC for 15k instead of the market price. John is happy because he made 4k on that deal.
This explanation is still pretty rough and it doesn't count many other circumstanses (or I'm simply mistaking something) and everything may vary depending on the asset, exchange etc. But this is how the general futures contracts should work.