Is backing out of an agreement to pay an agreed-upon sum to a broker, and reducing it before the contract is finalized, considered devouring people's wealth unjustly?

The broker is entitled to his full commission after completing his work. If the custom in your country is that the broker has completed his work merely by showing the land, then he is entitled to the agreed-upon commission. However, if the custom dictates that his work is not complete until the contract is finalized, and you withdrew before the work was completed, then you owe the broker a commission commensurate with his work, as determined by custom.