Sellling with owner financing, amortization question

[Logan Turner]

So I sold a property that didn't align with my portfolio with owner Financing. Purchased for 41k, 16k rehab. Sold 99k with 5k down, 10 year note at 10 percent interest. Payments around 1250 a month. So, the lawyer who I used to draw up the contract and close, said the lender has to let the owner pay down the principle sooner, if the owner wishes. I initially negotiated with buyer that interest rates would be fixed and built into the loan so total amount due would be 155k (no matter when it was paid off) I guess this is not allowed? Perhaps I should have just made the sell price for 155k paid over 10 years. Now I have a loan amortization schedule. Do you all use a bank to manage that? I don't wish to constantly recalculate numbers if they pay me 100 dollars extra one month, etc.. In the future, what do you guys recommend doing to avoid this? Also, this may be my last sale for a while lol. My goal is long term wealth building, I just thought this was a good opportunity to play bank and not worry about taxes, insurance and maintenance / vacancies. ThAnks in advance

[Logan Turner]

In case anyone was interested, i found a template for Excel here  

http://download.cnet.com/Amortization-Schedule-for...

and for numbers here https://jmoore.me/2013/10/31/a-loan-amortization-w...

I ran across a couple of programs for around $100-150, not sure what they entail but the free ones seem to work out great so far