We are a manufacturer's rep for commercial HVAC equipment - meaning we purchase from the manufacturer and then ship it to a job site. We bill the mechanical contractor who ordered it from us and they do the installation (set the unit, piping, etc.) and then part of our obligation is to provide a factory authorized company to officially start up the unit, who we hire and pay as a subcontractor. Question is - when we bill the mechanical contractor for the unit, they are holding 10% retainage on our invoices. Note that we do not bill the entire amount of our sale to them (for example, if the unit is $25,000, we bill only $24,000 and leave $1,000 balance to bill when the unit has been satisfactorily started up. Do they have the right to hold this 10% of the $24,000? If not, can I get a copy of the law that addresses this? Thanks!