If the Item no. in your Intercompany companies aren't the same, it is recommended to use Common Item no. for mapping Item numbers.
Give your Item a No in Common Item no. in both companies.
Partner card setup
On The IC Partner in SALES COMPANY, “Outbound Purch. Item No. Type” should be set to “Common Item No.”.
On The IC Partner in SUPPLY COMPANY, “Outbound Sales Item No. Type” should be set to “Common Item No.”.