If you have a contract between you and the client, this will answer your question. Does the contract list the entire price as $180, or $18, or some other amount? If you have delivered against milestone one and there are more milestones, then the client has simply slowed the effort. If the contract was for $180 with only one milestone funded, for $18, ask the client to set more milestones and fund them or you will invoice him for the remaining $162. If the documented contract was for $18, you may be out of luck.