In 1963 Gustaf Jonasson published a long article called Karl XII:s baltiska militärpolitik under 1701. In it he attempted to determine when the idea to move the main army back from Courland to eastern Livonia in preparation for an attack on Pskov was abandoned. Jonasson also discusses the reasons behind the changed plans and suggests that it had to do with a request for help from Hetman Sapieha. It arrived in early September and resulted in the King sending Colonel Hummerheilm into Samogitia with a small cavalry force. At about the same time there came a letter from Prince Jakub Ludwik Sobieski, who asked the King to help the Sapiehas.
According to Jonasson this letter of recommendation was likely a result of a visit Herman Sapieha had made to the Prince's estate at Ohlau (Oława) in August. Jonasson bases this on Carl von Rosen's Bidrag till kännedom om de händelser.., vol. 1, page 18. The source given by von Rosen is a letter from Georg Wachschlager to Mauritz Vellingk, dated 27 August 1701. This letter can be found in Riksarkivet (Polonica, vol. 117). So what does it say? Apparently Jonasson never checked...
The letter is in French and the relevant part is like this: "Il m' dit aussy que le Marechal de Lituanie Sapieha etoit arrivé en ces quartiere pour luy parler des affaires de consequence.." (He also told me that Marshal Sapieha had arrived in these quarters for talks with him on important matters). Carl von Rosen apparently did not discover that there are more letters on this subject. On the same day Wachschlager also wrote to Charles XII (Polonica, vol. 110), reporting: "Er vertraute mir auch dass der Littauische Marschalck Sapieha dieses Orts ankommen und mit Ihm wichtige dinge abzureden hätte". And on 3 September to the King: "der Littauische Grossmarchall Sapieha ist noch dieser Orten und gegenwartig bey dem Königlichen Printzen Jacob zur Olau" (The Lithuanian Grand Marshal Sapieha is still in these parts and right now at the Royal Prince Jacob's estate Olau").
So Marshal Sapieha... Is this really referring to the Hetman? No, he would have been called "Feldherr" or "Grand General".
Wachschlager instead means "Le grand maréchal de Lithuanie" ("Marzałek wielki litewski"), i.e. the Hetman's son Aleksander Paweł Sapieha (1669-1734). Later letters makes this very evident. On 7 September Wachschlager writes to Charles XII that the Hetman's son the Grand Marshal last Sunday met with the Prince.
A small detail, but it shows that you need to look at the documents and not just take some previous historian's word for granted.