What makes a promise promissory?

The Lutherans I’ve dealt with stress the promissory character of the gospel. I wouldn’t’ have a problem with this were it not for the fact that they define a “promise” in very eccentric terms. They act as though something can only be a promise if it’s unconditional.

Now, suppose farmer tells his 10-year-old son that if his son completes his chores, dad will take him to the zoo on Saturday.

Is that not a promise?

Suppose his son refuses to complete his chores–as a result of which his dad won’t take him to the zoo on Saturday.

Did the father break his promise? No.