Cash on the barrelhead is a phrase that refers to immediate payment for goods or services rendered. However, there are other synonyms for this phrase that can be used interchangeably in different contexts. One such synonym is "upfront payment," which means that payment is made prior to receiving the product or service. Another synonym for cash on the barrelhead is "cash in hand," which refers to physical cash being exchanged at the time of the transaction. Other synonyms include "immediate payment," "on the spot payment," and "payment in full." These phrases are often used in business transactions and negotiations to convey the importance of timely payment.