Wait didn't this all start because Kagerou was hungry? Why trade the food for rocks, and to the person who gave her the food in the first place?
The Silent panels might be confusing but this is how it went.
Kagerou was hungry and kinda dazed. She bit Wakasagihime, and then she ran off panel and came back with a basket of food FOR Wakasagihime as an apology. She wasn't bowing her head to thank her for food. Kagerou was bowing her head in addition to giving the food as an apology. This prompted Wakasagihime to get some jewels as "payment" for the food. Kagerou wonder's if Wakasagihime is alright fromt he bite and well, the rest is history.
You may ask "Why did Kagerou bite Wakasagihime if she seemingly had access to a basket of fruits?". To which my answer is that she might be away from her cottage/home and she noticed a little fishy tail. It's basically "Food now" vs "Food later". Kagerou thought it was just some big fish, not a mermaid.
