Down payment with Seller financing

[Jeffrey A Hayes]

Hello everyone, 

I have my eye on a duplex and I'm about $10K short for the 25% down typically required for a 2-4 unit property.  I've contacted the seller, and they are open to offering seller financing on a portion of the house.  Question is, how much do I take advantage of it? My initial thought was to only ask for the $10K but then if I did $20K, that would leave me with some cash reserves for the just in case scenarios...or eventually a shorter leash on another property. 
Has anyone structured deals like this and will the bank have any problems with this assuming they know where the money is coming from?Thanks!