Contracts that provide for installment progress payments that are based upon an estimated percentage of completion, with a percentage of the contract's proceeds to be retained by the Town pending completion of the contract, the retained amount of each progress payment or installment shall be as agreed-upon by negotiation, but not less than 5%. When the work to be performed on a Town project by multiple prime contractors or by a prime contractor and multiple subcontractors, the work contracted to be done by each individual contractor or subcontractor may be considered a separate division of the contract for the purpose of retention. As each such division of the contract is certified as having been completed, that portion of the retained funds which is allocable to the completed division of the contract may be released forthwith to the prime contractor, who shall, within ten days of its receipt, release to the subcontractor responsible for the completed work the full amount of any retention previously withheld by the prime contractor.