Unless you have a union agreement or employment contract with an outline of your specific duties, then you are an "at will" worker. This means that your company can set the conditions of your employment much as it sees fit (absent some form of actionable discrimination). Accordingly, as a sales associate you can be required to perform management functions as part of your regular job. If you find this to be unacceptable, you can either refuse these duties and risk termination or you can quit. Those are your only options.