Probably not, but you wouldn’t want to have to litigate it to prove it. 

As a general rule, a contract is not binding if there is no agreement on its material terms, and compensation for an employee would normally be considered a material term.  In addition, the minister is probably aware that salary has to be approved by the congregation and that the board doesn’t have authority to agree unilaterally.  If you ultimately failed to agree with the minister on salary after having “promised” to retain him, you would probably win a lawsuit.  But why put yourself in position to face such a suit?

Your board can tell him it wants to retain him after his current arrangement expires, subject to an agreement on the compensation and other terms.  It is in his interest as much as yours to know whether the salary will be satisfactory to him.  You have a year to work it out and get the approval of the congregation.  But it doesn’t make sense for either of you to make a commitment without first agreeing on the critical terms.