RE: Contract for Deed-- it is the same with a mortgage by bank.

In a contract for deed, the holder in essence merely has a first lien, so a buyer cannot sell off any, or put another lien on the property---- unless the contract for deed allows it, and many do. 

Typically there are restrictions in how far the buyer can go, and the contemplated lender would know those restrictions because such would be in the abstract........so most lenders would not do that, even if the contract allowed it, merely because of being in a position of having to pay off the contract in order to get first position for collection.

A bank holding a first mortgage is in the same situation.  If an owner wants to sell off a piece or two, then the bank has to agree with it as the primary lender, and release their mortgage on that portion.

A well-written contract is nothing to be afraid of--------- the typical hicky is when the attorney does not get the deed signed and included in an escrow, so upon complete payment, there is no problem in finding someone who can pass a clear title.