@boomerius, thanks for the question. It's an interesting one.
One way to do this would be as follows:
1. Create a temporary envelope called, for example, "Loan from Mike" and set the budget to be the amount you received, say $100.
2. Then, use Record Expense with the "Credit/Refund" option to put the money that you received into it. Again, $100.
3. I'm guessing that you're going to need to spend that money for something otherwise you probably wouldn't have borrowed it. To track this appropriately, you should do an Envelope Transfer from "Loan from Mike" into another envelope, say "Eating Out" and do your actual spending from that Envelope (i.e., when you record an expense at a restaurant, record it to the "Eating Out" envelope).
4. When you have extra funds, or when your friend starts knocking at your door, you'll do an Envelope Transfer *back* into the "Loan from Mike" envelope. Or else fund it from your paycheck, for example. Your budget on the "Loan from Mike" envelope will tell you when you have enough in there to repay him.
5. Finally, record a regular Expense transaction to repay Mike.
In step 2, I suggest using "Credit/Refund" rather than "Add $" to keep your spending reports straight--you're not actually spending money in the category "Loan from Mike". The way I'm describing it, your initial Credit/Refund will be offset by the Expense transaction in step 5 and will zero out your "spending" in that Envelope.