OWNER FINANCING Deal - Being the Bank

[Mitchell Benson]

Hello! Currently working on closing a deal with a wrap mortgage. I owe 32k, sales price is 109k with $9,000 down and finance over 15 years. at 6.5%. This is raw land that is mostly in the flood plain so i am happy with it. 

My question comes in to writing the documents for this. We have an attorney that we will be closing with but i want to be sure to have things written in there like late payment fees, ect. My real question is, is there a spreadsheet out there than can calculate late payments, added interest and lump chunks of cash thrown at the loan.

for instants, his payments will be $871 per month, so how can I keep track of the amount that is due on the loan if say, he misses a months payment but pays $1,300 one week and $300 the next. I guess what i'm getting at is how do i keep track of odd payments and still have the correct amount go towards the principal and interest respectively...

thanks for the help!