Do I pay Zakat on money someone owes me and has not yet paid?

When someone owes you money there is a rule for good and bad debt. Depending on the situation you would either be liable to pay Zakat on it or you may be exempt. 

 You are liable to pay Zakat if you are certain that you will be paid back the money that is owed to you. This is classified as good debt and you would include this amount towards your Zakat calculation. 

You are NOT liable to pay Zakat on any money owed to you if you have any doubts about receiving the money, this can be for many reasons but if you are uncertain about collection then Zakat is not due.