Consider the context of the dialogue - what leg could they mean?
Mr Sage could be a person. There is nothing to tell you he is not. He may be the bank manager who has loaned money to them. He could be their landlord. What is clear is that he owns a part of the furniture.
My understanding, given the limited context, is that everything Aunt Jane had, the car, the radio, the piano, and the furniture was bought on credit (See intallment plan), and none of it has been paid for, so it doesn't belong to Aunt Jane, who appears to be deceased, or to the relatives, possibly person A, who will inherit her belongings. It all belongs to Mr Sage, my guess, the man with whom Aunt Jane set up the installment plan.
As for the leg, it could be in reference to several things, one of which being a prosthesis, person A's:
B : I suppose all you own is the prosthesis. (she points to one of person A's legs)
A : Well, no, as a matter of fact, it's that one. (she points to another/ the other leg).
As for this, here,
B : I 'm not going to sit on Mr. Sage's part for anyone. (She stands up. Now, tell me, how much do all these instalments come to?)
Seems to me that it refers to Mr. Sage's part of the inheritence. Person A wants to settle her Aunt Jane's debt.
Additional context would help, especially with the "leg" part.