problems with Purchase Req.no in relation to Third-party-case

Hi,

Im trying to create a third-party-case invoice with Sales Order No. xxxxxxxx

I'm getting an error regarding missing Purchase req. no. (the customer's credit block had to be released).

This concerns a product that we buy from Helsingborg as XXXX1 and we sell it in XXXX2

one more question?

From where the valuation price for PR (w.r.to SO) being generated is taken?

Thnaks in advance

Akkshaya