Word choice: back or ago in [three payments of ... back/ago]

Suppose that John lends you some money to help you run your new small business regularly. He sees a special edition bill in a picture frame on the wall of your office.

(1) John said, "Where did you get this special edition bill?
(a) I said, "It came from four payments of your financial aid back."

(b) I said, "It came from four payments of your financial aid ago."

(c) I said, "It came from you four payments of your financial aid back."

(d) I said, "It came from you four payments of your financial aid ago."

My focus is on the "four". I got the bill not from the last payment, not from the payment before that, and not from the payment from the previous one. It came from the one before "that" previous one mentioned.

I hope you understand what I am trying to say. Most of my non-native English speaking friends think either (c) or (d) is correct with "back".

Do you think my friends are correct? Please help me. Thank you very much.