Morris v. Woodside

Is the following real estate contract due-on-sale clause enforceable: "During the term of this Real Estate Contract between seller and purchasers [sic]. Purchasers agree not to sell said premises without paying off said Real Estate Contract."?

The facts: In 1978 plaintiff/appellant Morris, as purchaser, entered into a real estate contract with defendant/ respondent Woodside, as seller, which contract contained the above clause. The contract concerned the sale of residential property on Vashon Island for a sum of $65,000. In 1980 Morris conveyed the property to defendants/respondents Slichter…