Can a FSBO sell a home under a land contract if he has a HELOC on the property?

Looking to purchase a home from a FSBO who has a HELOC he is telling us that he can lock the interest rate in at 4.5% and give us an interest rate of 5.8% to cover the HELOC ... im asking if this is even possible since by filling it it would make the HELOC payable at that point because of the due on sale clause... he is saying that this is not the case and that the bank told him once he locks it in he can sell the property without the due on sale clause coming into effect. Is this true?