FACTS: SFH

FMV $650k

ARV $700K

1st $430k

2nd $ 51k (HELOC max line is $350k)

possible rent $3200-$3500

mtg payments $2,800

Seller is open to long term master lease or sub-2

Deal is not suitable for a quick flip.

Question: How can I control the property & keep the exiting HELOC in place but prevent the seller from utilizing additional credit from the HELOC?

Get the loan docs for the HELOC and see if it allows for the draw period to end and the repayment to start. You might not like the fully amortizing repayment numbers though, but for that balance it might not be too painful.