Seller Financing Question

Hello,

I'm a buyer and the seller is willing to do seller financing. 

Purchase price $300,000

Downpayment $25,000

I will pay the seller $5,000/mo for 1 year (while I rehab the house) and then balloon payment at the end of the year when I sell the house and it closes. 

$300,000 - $25,000 - $60,000 (12 x $5,000) = $215,000 

Balloon payment $215,000.

The seller still has a mortgage and pays $1,500/mo.

Should the monthly payment that the seller maked during the year be added to the balloon payment? $1,500 x 12 = $18,000.

New balloon payment would = 215,000 + $18,000 = $233,000.

I'm not sure if it should be included or not. Thank you for your help in advance. 

Franck