Change constraint on column based on value of another

Is it possible to change a constraint on a column in postgres based on the value of another column?
E.g. (pseudocode):

CREATE TABLE transactions(
    id SERIAL PRIMARY KEY,
    type TXN_TYPE NOT NULL,
    amount BIGINT,
    . . . .,
    refunded boolean DEFAULT FALSE,
    refund_id DEFAULT NULL if (CONSTRAINT link_refund CHECK (refunded=TRUE))=TRUE REFERENCES transactions(id)
);