As long as your reasons are specific and in writing, your supplier will be forced to respond to you if he wants to get paid.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We regret that we must withhold payment of <reference invoice> until <conditions for payment>. <Elaborate on reasons.>
Until <conditions> we must continue to withhold payment. Upon satisfactory completion we will be happy to pay you in full.
Thank you for your prompt attention to this matter.
encl: <List of enclosed items goes here>