No one can serve two masters. If you try, you will end up loving one and despising the other. You cannot serve both God and money - You must choose.
Here the word for money in the King James is "mammon," which means wealth or treasure and, specifically, riches when it is personified and opposed to God.
What is wealth personified?
When our interaction with wealth becomes a relationship, to which we answer and serve. This happens when we have an intimacy with wealth, preferring it and being ruled by it, rather than seeing it as a tool. This relationship becomes a barrier to our relationship with God when it becomes a master relationship - where we serve wealth and its purposes.
What is wealth's purpose?
To love it (and not God), to let it be an idol (main focus) to the point where other parts of life are sacrificed to its pursuance is sin's purpose for wealth. When we use wealth to dominate and influence others, thus proving its high status in our lives, this demonstrates that it has gotten out of place.
What is God's purpose for wealth?
Wealth's purpose is to serve God and to take care of others with it. Our time is not to be overused simply in the accumulation of wealth. We are to balance our time, God getting preference over all. Most of all, we are to remember who we are to serve with our being and our efforts - the God of the universe. He provides. He may use us to provide for others. We must remember that wealth is a tool and not a toy.