Can I have a set principal and interest payment in a note ?

[Ray Slack]

I'm buying a cheap property and the owner is holding the financing.   I proposed to the owner a payment of $100 per month principal and $25 a month interest with a 7 year balloon and she agreed.    Her attorney got ahold of it and is wanting to structure it as an amortization.  I can't see why we can't just do it my way. Much simpler.    Any law against just having set principal and interest payments in a note instead of a rate and amortization? This is in Maryland in case that matters.

Thanks