Harbormaster (Andrew) was not allowing us to pump out our almost full black water tank. I told him repeatedly that it may overflow at any time (a biological hazard). He said, unless we buy a full tank of gas, we are not allowed to pump out. The gas dock was completely empty and I was being accommodating to come into the dock at any time to avoid loss of business. I even offered to purchase some gas, just not filling up my entire 150gal tank. Plus, we were staying at a slip for two nights. I found this extremely unprofessional and frankly illegal, as it’s mandated by federal law to have free pumpout services; never in my 20yrs of boating have I had to purchase gas to get a pumpout. This policy needs to be changed and Andrew needs to read up on boating laws.