When using a main form and a sub-form, what ever fields on the two forms that are used to "tie" the two forms together will automatically be added to the record created by the sub-form. Of course those fields must be in the table that is used with the sub-form.
There is really no reason to have those fields show on the sub-form. They are already showing on the main form. To "tie" the main form with the sub-form, use the fields in the sub-form's property sheet named Link Fields Master and Link Child Fields to name the fields in the main form and sub-form that tie these records on the sub-form with the master record on the master form.

Now, about the additional buttons on the main form that will allow the user to create different kinds of records. May I suggest that you use a tab control rather than buttons? The the user can click on that tab which will show the form (payment agreement, discount agreement, etc.) that corresponds to the tab.