Unless it’s specified by your loan type or lender, you don’t have to.

Many lenders, even small ones, don't offer HELOCs on rental properties like yours.

Some rehab lenders have favorable refinancing programs to convert the rehab loan to permanent 30 year financing.

Or do you just not want to deal with lenders? 

Finally, as an aspiring/budding lender, I don't have as much access to mentors, but then again it's easier than rehabbing/flipping and probably easier than buying and holding and managing multifamily. 

This team consists of an Investor Focused Agent, Lender, Contractor, and PM.Read this article on the "core 4".

We do have a mortgage, so we would need to talk with our lender to avoid the "due on sale" clause (at least that's how it has been explained to me).

But if they don't willingly make the payment your only option would be to sue, and they have the upper hand to let the lender know about the sale possibly triggering DOS.

It could be a nice opportunity however I would be certain they will want you to make up the difference between $250,000 and whatever is left on the mortgage which btw in my opinion should be done with lender's knowledge and approval. 

Consultants, inspectors, and mentorsProperty managersAssistantsBookkeepers, accountants, and attorneysLandlords if you rent your office spaceYour business partners if you paid them (except if they were strictly lenders)Disclaimer about wholesalers: it is a controversial issue, with some of my colleagues claiming that wholesalers do not need 1099s.