How to reproduce:
Not sure if any mod causes this but I think no since I don't use a mod which changes money, diplomacy or trade except "Separatism" which allows kingdoms to separate into smaller ones.
- Declare war against a weak faction
- Get attacked by the enemy (outnumbered)
- Try to make peace by paying for it
- Offer 1 coin (trade icon will turn green)
- Demand for all the money of the lord (1 coin offer of player remains)
- Still green
- Accept
- Repeat this process