suggestions on how to present cash deal with delayed payment

[Randy Person]

I have an opportunity to purchase a 3/1 single family in my area. It is off market. Because of another deal I am financing I dont have the cash on hand to do the deal and while I have the equity to do a HELOC, I do not want to apply for the Heloc until my current deals funds. My seller seems amenable to my giving them 20% down which i have and the balance due in 120 - 150 days. Do I do a promissory note for the balance or how can I structure this?