The stranger says you can trust him with the money

Player: I want to roll insight

(dm hidden roll: deception: 15)

Insight 14 -> “you aren’t sure he’s lying, but you feel very uneasy trusting him”

Insight 5 -> “you really can’t read his expression”

Insight 16 -> “you aren’t totally sure, but you think he’s going to betray you”

Insight 19 -> “yeah this guy is totally going to rip you off”

The game not being interesting is largely a DM problem regardless of the framework you use.