Hi,
Hope you didn't post purchase order.
Just go to sales line table there is 4 column need to update.
1) Purchasing code = define as special order
2) special order tick automatically
3) Special order Purchase order no. = define your purchase order no. which you created against sales order earlier.
4) special order purchase line no. = define you item no. here i am sure once you define your po no. system auto give you list of purchase item line. then select your item line here.
it will link.
Please verify the reply if it answers your query
Best Regards,
Manan Shah