Owner Finance with existing portfolio loan

I have a rent house that I am selling and have a buyer interested in owner financing, but I have a 15 year portfolio loan with 2.5 years left for the balloon. He wants to put down $50,000 and I only owe $60,000 but I’d like to not tie up that amount of money for very long. I will try to get the owner finance balloon at 2 years, but if I can’t what are my options? How can I avoid the due on sale from my portfolio loan? Are these more or less likely to be called DOS? (Do i need to do a land trust? How?) If he needs longer than 2.5 years how would I refinance the loan when he’s in it?

Thanks for your help!

Also, how will that affect my DTI, etc when getting a conventional loan on a new primary residence?