In Go Down, Moses, McCaslin Edmonds tells Ike, "So you have plenty of coppers anyway. But they are still not enough to be either rarities or heirlooms. So you will have to take the money." Is this a ruse to distract Ike and keep his mind off the "silver cup"? Ike replies, "As a loan. From you. This one," to which McCaslin responds, "You can't. I have no money I can lend you." What money are they referring to—the inheritance money borrowed off the value of the plantation?

To answer the first part of your question, McCaslin Edmond's question is likely not simply a ruse or a distraction. This section of the story is part of a very long dialogue between McCaslin Edmonds and Isaac, a.k.a. "Ike," McCaslin about inheritance.
Ike is supposed to inherit the McCaslin family fortune and plantation. He is also supposed to inherit a legacy, a silver cup full of gold pieces, from his mother's brother Hubert Beauchamp. Hubert supposedly wraps the cup in a burlap envelope (see page 288) for Ike to open when he turns twenty-one. Occasionally he takes the parcel out and passes it "from hand to hand . . . insisting that each take it in turn and heft it for weight and shake it again to prove the sound" (290). There is a lot of build-up to this inheritance, and Ike is remembering this at this point in the story (Faulkner makes use of analepsis quite frequently).
By the time McCaslin tells Ike the coppers aren't rare enough to be heirlooms, Ike is twenty-one, and his conversation with McCaslin is about repudiating his birthright and family fortune/plantation, and giving it to McCaslin instead. McCaslin doesn't want him to repudiate. He thinks Ike, as the patrilineal heir, is the rightful one. McCaslin is only heir on his mother's side, and in a deeply patriarchal society, he sees Ike's repudiation as shocking.
But now, Ike's other inheritance, the silver cup full of gold pieces, is a sham. His uncle, who lived on a plantation without slaves to work for him and who refused to work himself, spent the fortune that was supposed to be for his nephew and filled the vessel with IOUs. He even sold the silver cup—the last IOU is "One silver cup."
With no gold pieces, McCaslin tries to convince Ike to take his birthright inheritance: "So you will have to take the money."
Ike still refuses. He would only take money if it were a loan. So, to answer the second part of your question, the money is actually Ike's rightful money in his bank account, likely the same account he uses to transfer money to his family's former slaves. But because he has repudiated this fortune, and McCaslin is reluctant to accept it, McCaslin says he has no money to give, but Ike can go to the bank and get it for himself.