“If you start out by promising what you don’t even have yet, you’ll lose your desire to work toward getting it" (25)
When Santiago was told to go find his treasure, he met an old man. That old man, King Melchizedek, asked Santiago to give him one-tenth of his flock if he wanted to know the way to finding his treasure. However, Santiago suggested that he'll give him one-tenth of his treasury instead. This is the part where King Melchizedek advised Santiago to not make a decision on something that he is not sure that he'll have a possession of.
This type of situation can happen to a lot of people. In some case, although, a person may not be sure whether they can do a certain thing, they'll make a promise that it'll come true or can be done.