A Promisor is the person who is responsible through a promise (typically written in a contract) to another person.
One that makes a promise.
Duhaime Legal Dictionary
The person who has become obliged through a promise (usually expressed in a contract) towards another.
Lect Law Library
The promisor is bound to fulfil his promise, unless when it is contrary to law, as a promise to steal or to commit an assault and battery; when the fulfilment is prevented by the act of God, as where one has agreed to teach another drawing and he loses his sight, so that he cannot teach it; when the promisee prevents the promisor from doing what he agreed to do; when the promisor has been discharged from his promise by the promisee, when the promise, has been made without a sufficient consideration; and, perhaps, in some other cases, the duties of the promisor are at an end.