I don't think you can use the list function like this, to concatenate two columns together. You can use the list function to get the value of a calculated field that concatenates the paymentand the commission percent.
The other option is to build the list through a script and present that in a text field or by other means.
You'd find all the payments of a sales person. Go to the first record and get the payment in a variable called $Payment. Then get the calculated commission percent and set variable $Payment = $Payment & [calculated percent]
Then set a variable called $List as List ($List ; $Payment).This, of course, will add the value of $Payment to the end of the list.
Loop through all the payments of that sales person and then finally set a field to the contents of $List.
Is the Salesman's commission rate is stored in the Salesman table? If so you have access to it via the Salesman - Order relationship.
Order::Commission = Order::Amount * Salesman::commissionRate
You can then use a summary field that is the Sum of Commission to create a report of the commission due.
I'd say wrong direction. You probably need to do some table restructuring. It depends on whether Payments apply to more than one invoice and can multiple salespeople draw commission from the same order. It's hard to tell what tables you already have and what your requirements are.
But possibly not, what about storing the commission percentage in the Payment table as an auto-enter calculation? Do your commission calculation in Payments and report on that table using Summary fields.