Be careful of assuming that it is money. Examine the perceptions of the other person . Remember, perceptions are neither right nor wrong, they just are. Your job as a manager is to make sure that you and your employee see the world the same way. To be able to do this, you have to be open and you have to listen.
Good communication is the key.
Examine expectations . No one comes to work with the idea that they're going to be demotivated. They come with great expectations about the job. Every time one of those expectations is not met, the employee loses a little motivation. This is particularly relevant for rewards.