Edit :
Federal reserve notes are legal tender, meaning that the failure to accept them as repayment of debt results in a default on your contract, by operation of law. So yes, there IS a compulsion to use federal reserve notes.
Actually, this is wrong (far be it from me to tell you how to do lawyering) but contracts can be done in any currency trading parties choose, so there is no compulsion just an implied contractual arrangement that you are using legal tender (which happen to be private FR debt notes) when it is not explicitly stated in a contract otherwise. Just having a SSN implies an unwritten contract to use FRN private debt notes .... this is how they get around the constitution by using contract law (implied consent to contract when choosing to be paid in "legal tender" etc, etc) and the body of contractual law surrounding private debt notes.
Legal tender laws don't apply to goods and services. They mainly apply to pure loans. There is no legal obligation to accept cash for goods and services, but there is one to accept it as repayment for a loan (as distinguished from sale on credit). However, given the prevalence and necessity of loans for almost any kind of commercial enterprise, there is definitely a compulsion to use federal reserve notes.