69 Pa. Stat. § 1802
When a subsequent purchase is made, the entire amount of all payments made previous thereto shall be deemed to have been applied toward the payment of the previous time sale price or time sale prices. Each payment thereafter received shall be deemed to be allocated to all of the various time sale prices in the same proportion or ratio as the original cash sale prices of the various purchases bear to one another; where the amount of each installment payment is increased in connection with the subsequent purchase, the subsequent payments (at the seller's election) may be deemed to be allocated as follows: an amount equal to the original rate, to the previous time sale price, and an amount equal to the increase, to the subsequent time sale price. However, the amount of any initial or down payment on the subsequent purchase shall be deemed to be allocated in its entirety to such purchase.
69 P.S. § 1802