It seems unlikely that the good Defense Professor would have orchestrated a plan which is entirely dependent for it's success on Lucius failing to take advantage of the situation - unless putting Harry in Lucius's debt was his goal.

Assuming Quirrell is Voldemort, he presumably had years of access to Lucius' mind (if he regularly required Lucius to drop Occlumency barriers). At the very least, we can assume he has an excellent mental model of how Lucius behaves. The plot therefore doesn't seem like too great of risk for Quirrell, particularly when we consider that Lucius is about to discover Harry's progress in turning Draco. Quirrell can safely assume that Lucius will react in a way that will pull Draco and Harry apart when he discovers this, and will therefore be less inclined to trade Hermione for something of Harry's.