So-called "owner financing" usually applies to owner who owns the property free and clear. In your case, you still have a mortgage and cannot pay it off any time soon. Therefore your currently lender is still the beneficiary of the loan-you ain't finance anyone.
Tenants proposed owner financing usually indiciates either they have trouble getting conventional loan(no adequate downpayment or income does not qualify)or they are hoping owner provides more feasible interest rate.
If your tenants did not do their homework(seems to me they did not),they might just hope that they can purchase the property and pay the same mortgage payment as their rent. Ideal situation for tenants: no extra $$ out of their pocket, paying similar monthly and own the home. But most of the times, things don't happen as we all wish!?