Hi Dama,
The very reason that there are two payment terms fields in the customer master is so that sales and finance can use it independantly of each other. The customer is created centrally (XD01) with the idea that both, sales and finance teams are working together while creating the customer master. Had this not been the case, i.e. if the teams were working independantly of each other, the sales team (VD01) and the finance team(FD01) would have created the masters at their ends and would have selected the terms of payment that each desired for their part of the business.
Sales order is always a transaction from the Sales end. That is why the terms of payment that get picked in the sales order is always picked from the sales area data tab of the customer master.
Regards,
Sumita Mittra