Let's say I were to write an IOU as a joke to a friend for a large sum of money (eg $5000) and I signed it, but my friend did not borrow me the $5000. Also, on the IOU there are no specifics on how and when I would repay him back.
Your question is whether a borrower would owe money to a lender who didn't lend them money, but with whom the borrower has a signed promissory note or loan agreement. In a nutshell - no, they would not.
To being with, a loan is a contract. Whether written or oral, a contract consists of 3 parts: Offer, Acceptance, and Consideration. In your situation, that means the lender must disclose what they are offering (i.e. how much money, what interest rate, what repayment terms, etc.); you must accept agree to be bound by those terms; and the lender must then provide what they promised; the money or credit. Failure of any of these 3 means that no contract was formed and there is no obligation on your part. In short, if you didn't borrow money, accept credit, or instruct the lender to pay a debt on your behalf, what is there for you to repay?
Keep in mind that this answer is based on my understanding of whatever facts you provided (in this case, not many). I would not depend on this answer alone in resolving any issues you have; rather I would use it as a guide and talk to an Attorney in my area who is knowledgeable about these kinds of transactions and about the details of your situation.
