I want a function to make a calculation in another function C++

I try to make a function in C++ to assign a value to a variable and keep it, But I want to make with void function. This is the Main function;

int main(){
    //declar variables
    double sales=0.0,commission=0.0,totalCom=0.0,totalSales=0.0,totalEmployee=0.0;
    //call soldEmployee function
    sales = soldEmployee(sales);    
    // if use enter sales negative the programe end
    while(sales > 0){
        //call calc commission
        commisionCalc(sales,commission);
        //call display commission
        displayCom(commission);
        //call display total (commission + sale)
        disTotal(sales,commission,totalEmployee);
        // ask user to put input again
        sales = soldEmployee(sales);
    }
    total(totalSales,commission);

    return 0;
 } //end of main function

And this the function;

double soldEmployee(double &s){//function to get the user input
    cout << "Next Sales: "; cin >> s;   
    return s;   
}

void commisionCalc(double s, double &com){//function to calc commission
    com = s * 0.10;
}

void displayCom(double c){;// function to display commission
    cout <<fixed << setprecision(2) << "Commision= " << c << endl;
}

void disTotal(double s,double com,double &total){// function to get commission + sales
    total = s + com;
    cout << "Salary + Com: " << total << endl;
}

void total(double t, double &total){// function to get the total of all employee commision and store it in total sales
    t += total;
    cout << "Total Salary: " << total << endl;
}

So, in function total I want to make it assign the commission I get into the totalSales, I know that the name of variables seems confusing but, this is because I made some modifies in the program. Can anyone help me explain how to make because I stuck when I made many function. and if you asking that there is a easy way to make i want this way because it's a assignment not work to learn how to make function and get used to it.