Regarding the relationship between Marni and the merchant, Rokhan knew this by accident.
At that time, Rokhan intended to sell the medicinal plants he had managed to collect to the merchant. However, Rokhan only saw the figure of Aryan playing top with a slave, in front of the merchant's stall.
Then, where is Marni or the merchant himself? This is unusual, Rokhan thought at the time.
And yes, out of curiosity and suspicion of something, Rokhan turned around to approach the merchant's house. And when Rokhan was near the storage shed, that's where Rokhan saw the merchant having sex with Marni who was bending on a pile of grass nearly naked.
Since then Rokhan intends to be able to do the same thing to Marni. The admiration and love he had for Marni turned into something bad.