When you get a new plane, you pay a certain number of months in advance (Saftfrucht say 10% of the total time).
Anyway, even if that money appears as negative in your balance, you don't really pay it (because you already paid). If you look at the Cash flow statement, you'll find a row that says "Adjustments of items where a payment is not included". There you'll find the "Aircraft lease prepayments" with the amount of negative money in you balance.
So, at least from the leasing cost point of view, it makes not difference in money.
My advice is to make the Piper fly until you can cancel the lease.