Hi Ristesh,
I have few questions regarding your scenario:
1) Do you want the unrealized Forex Loss posting to be done at the time of principal decrease.
2) Why you are using securities for interest rate instrument transaction (borrowing in your case), thats why system is calculating the negative purchase value.
I tried same scenario in my system and it worked fine. Please see:
Let us know your view on this.
Cheers !
Aditiya