Bad payment

They call me out from Pretoria for a problem on a ultrasound unit. The unit keep give them a error code. I advice that the problem normally for those type of error codes is. it could be the power supply or monitor. I replace the monitor with a refurb one and problem solved. The called after a week and say they do not what to spend that amount of money on the sonar. I must bring back the old monitor witch I already repaired as refurb one and just set out of focus. So I take it back. I send them a invoice for travel and travetime and labor. Total invoice R3400 +/_ Now they refuse to pay because they said I did not do any thing. Well I did. But still they call me so they still have to pay for my travel and time to take every thing back. After they used the repaired monitor for a week. They also gave me a second sonar too repair and I was waiting for part that I order for it and they just wanted that back as well.