Hello Renatas
I also had doubt on the same. I did short transaction future derivate. Transaction Type 400 (open transactions) and Flow Type 1510 (Sale Future). I did at $ 25.
Now my last market price fallen to $ 20. So it means I have gained $5 (since its short position).
I maintained this last market price in SAP against closing price.
When I ran TIS1 (position report), system showed me loss of $5. But actually its gain and its showing as "Asset" under Asset / Liablity Position Indicator. Please help how to show it as gain.