Tenant prepay lease 10 years!!!!

[Spenser Harding]

I have a small multi I am looking at purchasing where an older woman with M.S. wants to prepay 10 years of rent for $40,000 upfront. She wants the simplicity and stability over the long-term. She likes the place and wants to stay. Market rate for the unit she is in $500/mo. The $40k would finance a portion of down payment instead of taking it out of a HELOC. All things equal it looks like I would be losing $20,400 of potential rents as well as rent increases however I would have 0% vacancy in one of the units. Further, I would theoretically pay $26,000 of interest over the ten years if a) interest rates stayed the same on my adjustable rate HELOC b) I never choosing to pay down the principal on that portion of debt. Finally - this would be state allowable and a lawyer drawn contract. Feedback appreciated!