The sub-form is a form. (No, I'm not being off-handed here.) It has all the abilities of any other form. Including an implied order of reporting.
So you can open the form directly outside the context of the form that calls it. And you can set an OrderBy on the form when in that context. Save it that way. You would never USE it directly because it doesn't yet have the filter for the person in question. You would get payments in date order for all persons, which would be a mess to look at. But you wouldn't open the form directly that often anyway.
Once you establish the order, when you open the form in the context of a Subform, the fields on the sub-form control (which is part of the parent form) still limit the fields you retrieve - but the order specified in the subform itself (the thing that occupies the sub-form control) will be the order you specified separately as I described earlier.
In other words, you specify the FILTER by the properties of the sub-form control on the parent form. But you specify the ORDER on the actual subform that fills the sub-form control.