Pidyon Haben Coins

Question

I bought pidyon haben coins and gave the seller the first of five installments of a mutually agreed upon payment plan. ( The full amount will not have been paid at the time of the pidyon. ) Am I considered to be the "owner" as far as the halacha of owning the coins is concerned ? Or must the full amount be paid prior to the pidyon ? The pidyon is this Sunday 11-7-04, so please respond as soon as possible. Thank you.