Lets say I open a first position for 10usd then second position I want to open is 20usd (while the first positio is still running)
I select 20 usd in the exchange for the second position and the exchange opens a position for 17 instead of 20 for example..
I am using limit orders and is not related to the minimum amount to be traded
This is how exchanges are. You will noticed that the amount you use to open a position is also changing. Sometimes it will increase and sometimes it will be decreasing but depending on if you are making money already or losing. You do not need to bother yourself about this. Just use an amount of money to open position and take note of your profit and loss, the trading fee and the funding fee after you close the position and use it to cekcjs
An example is when I open $250 long position, I can see $247 instead. Do not worry about it. I do not know the reason exchanges make it like that though.
I have actually experienced such a thing in the past when I was still very much active with futures trading, a couple of times I assumed that maybe the price of the asset I am opening a position on dropped a bit while the operation was going on, and with this, I never really paid attention any more..
I think the best thing to do is to find out from customer support why such a thing happens, it could turn out to be that the exchange takes fees for opening a position and also takes another fee when closing regardless of the outcome of the trade.
If then this be the case, then it's really shouldn't be, it's extortion in the highest order.