So 2 things happened:
At first the invading army asked for more money than I had (trading post and bank account combined). While this forced me into bad trades it also was an interesting challenge. So no big problem for me. But I thought it’s good to report it here because AFAIK invading army always will ask for money you can in fact pay instantly. Some abused this system and transfered all gold into the trading post. So maybe this is intentional?!
The second thing - and now it becomes really weird. I did some bad trades and gathered gold to pay the ransom. I accepted to pay, the invading army retreated but I kept all gold. So I did not pay at all but the system thought I did. For me, this is a new bug. In 0.7.4 ransom was bound to bank account (which could be abused as mentioned above) but there was no payment bug. Something changed here.