"Old Scratch" is just another name for the Devil, who initially seems to want nothing from Tom. He doesn't even bother to really identify himself, and never does he specifically say that he's the Devil by that name, nor does Tom call him such. However, once it becomes clear who he is, it doesn't take long for him to start cutting deals with Tom.
Tom, being a pretty awful person anyway, doesn't really have any fear of Scratch, and seems to get into the negotiations without any hangups about the basics of how these things usually go (which it is implied that the reader already knows about). The implication, which comes up twice but the narrator specifically glosses over as "easily surmised" and "need not be mentioned", is that the Devil requires Tom's soul in exchange for whatever favor is being asked of him.
In return, Scratch is offering Tom a lost pirate treasure that is under his protection and cannot be found (according to Scratch) except by those in his favor.
These basics are readily understood by both parties, although some details are negotiated later; for example, Scratch wants the money put to use "in his service", i.e. for evil; his first suggestion is that Tom operate a slave ship, but when Tom balks at this Scratch suggests money-lending at exaggerated interest rates, which is forbidden in the Bible.