We don't have synonyms for precontracted yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are precontract, preconstructed and precontracts.
(n) :
(law) A contract preceding another, especially a contract of marriage which, according to the ancient law, rendered void a subsequent marriage solemnized in violation of it.
(n) :
(law) A contract preceding another, especially a contract of marriage which, according to the ancient law, rendered void a subsequent marriage solemnized in violation of it.