Hi Dewi
Will this not work??
Try by configuring your payment term as above.. And use Factory calendar...
If the 31st day is falling on weekend, it will automatically adjust it to next working day
If you feel this wont help,, please give some clear 2-3 examples explaining your requirement
Nevertheless, you always have the option of using user exit in FI substitution to achieve somethinig not possible by Standard SAP
rest assured, your requirement has a solution possible!!
br, Ajay M