What do you do when a cab driver asks for a bigger tip?

Ask for tipping guidelines. If none are available, state that your company does not reimburse tips (many companies don't), so if the driver wants more money you need a new receipt with the larger base amount.

A tip is defined as: A gift Freely Given.  You cannot say to someone they did not give a big enough gift as the one receiving has no say in how big a gift is.  I would ask him to give me back what I gave him if he did not feel it was enough and leave without giving anything.  Any tip should always be accepted gracefully no matter the amount.  That is proper service personnel etiquette.  If they do not follow that the tip should be revoked.