Dunstan Cass is depicted as a drunken, selfish man, who has spent a significant amount of money that belongs to his father. When Godfrey insists that Dunstan come up with money to avoid their father's wrath, Dunstan threatens to blackmail him by disclosing his secret marriage to Molly Farren. Dunstan understands that the sensitive information will ruin Godfrey's chances of marrying Nancy Lammeter and eventually convinces his brother to allow him to sell his prized horse, Wildfire.

In chapter four, Dunstan accidentally drives Wildfire too hard, and the horse dies after being fatally pierced by a hedge-stake. Dunstan then heads back in the direction of Raveloe and notices a gleaming light through the mist as he nears the Stone Pits. The gleaming light is coming from Silas Marner's home, and Dunstan proceeds to walk towards his cottage. Dunstan originally plans on asking Silas for a loan but is astonished to discover that the door is unlocked, and there is food cooking on the fire. Dunstan thinks about the possibility that Silas is dead after falling in the Stone Pits, which explains why his door is unlocked and the food is still cooking. Dunstan then justifies stealing Silas's money by thinking,

If the weaver was dead, who had a right to his money? Who would know where his money was hidden? Who would know that anybody had come to take it away? (Eliot, 33)

Essentially, Dunstan justifies stealing Silas's gold because Silas is a single outcast and nobody would know the difference if he stole the dead man's money.

The robbery that you refer to occurs in Chapter 4, after Dunstan's plan of selling the horse belonging to Godfrey have come to naught when the horse dies in an accident. Making his way through Raveloe at dark, he comes across Marner's house, and is planning on intimidating Silas Marner into giving him some money and letting him gain interest from it as another alternative source of money. As he gets closer and enters the house, he is surprised to see that Silas Marner has abandoned his home with food cooking and the lantern on. Dunstan, perhaps looking for justification for the crime he is about to commit, wonders if Silas Marner had gone out to get more fuel and slipped into the Stone-pit:

That was an interesting idea to Dunstan, carrying consequences of entire novelty. If the weaver was dead, who had a right to his money? Who would know where his money was hidden? Who would know that anybody had come to take it away?

With this thought, Dunstan determines to find the weaver's fabled hoard and make off with it himself.