There may very well be a much easier method for this, but one method might be
to create a function that will find the Employee on the most recent Invoice
for a particular Customer.

Below is something very quickly compiled on the top of my head, and makes
certain assumptions regarding table and field names, etc.  You would then
simply set the DefaultValue of the text box to
"=GetMostRecentBill([txtCustomerID])".
-------------

Private Function GetMostRecentBill(ByVal lngCustomerID As Long) As String
On Error GoTo Err_Handler

    Dim db as DAO.Database, rs as DAO.Recordset
    Dim stSQL as String, stOutput as String

    stSQL = "SELECT tblEmployees.ID, tblEmployees.EmployeeName
Max(tblInvoices.InvoiceDate) AS LatestInvoiceDate FROM tblInvoices INNER JOIN
tblEmployees ON tblInvoices.EmployeeID = tblEmployees.ID GROUP BY
tblInvoices.CustomerID HAVING (((tblInvoices.CustomerID)=" & lngCustomerID &
"));"

    Set db = CurrentDb()
    Set rs = db.OpenRecordset(stSQL,,dbReadOnly)

    With rs
    If Not (.bof and .eof) Then
        stOutput = !EmployeeName
    End If
    End with

    GetMostRecentBill = stOutput

Exit_Handler:
    If Not rs Is Nothing Then
        rs.Close
        Set rs = Nothing
    End If
    If Not db Is Nothing Then
        db.Close
        Set db = Nothing
    End If
    Exit Function

Err_Handler:
    MsgBox Err.Number & " – " & Err.Description
    Resume Exit_Handler
End Function

----------------

Again, there is probably a much simpler method to accomplish this, in which
case I am certain that the Access gurus that reside here (and who possess
much more knowledge than I do) will happily offer it to you.

In any case, I hope this helps,
Jeff