ROI/IRR on seller financing. HELP

[Jesse Markus]

Hi everyone, I'm pretty new to this stuff but am learning everyday. Currently I am a general contractor for people who are BRRRRing. I just started so therefore i am self employed and cant get a mortage. Can't find a cosigner either. Here's my situation.

I found a property on the MLS for 78k. An investor/another guy i work for bought the property with plans to sell it to me on contract. He bought it for 85100. He gave me an offer to buy it from him for 97k with 7k down @8% over (however many years i want). We are talking pretty openly with each other to make sure each person is getting a good deal. This house is the cheapest house on the block and if i fixed it up (10k in) it would appraise for 140k. How do i calucate his ROI. If i calculate it as if he put in 20% down (about 17k) he would make a total of $20103 from the profit from selling it to me (97k-85k=$12,000) plus interest ($11875-$3772=$8103) The first number ($11875) is the interest i would pay at 8% til january 2023 which is when i can hopefully get a mortage. The second number ($3772) is the amount of interest he would pay on a $66000 loan at 3.5%, profitting him $8103 from June 2021 to January 2023. His total profit would be $20103 while only putting $17000 in. That gives him an ROI of 118%. I have heard about IRR and dont understand it. I understand that if i was doing a BRRRR or rental property that 118% would be a pot of Gold but how do i view it in light of seller financing/buyer financing? I need help understanding the math and help knowing what would be a good deal for both us. Thanks