The member of Shivaji’s Astha Pradhana who looked after foreign affairs was

In Shivaji’s Ashta Pradhan the Peshwa was the chief minister or prime minister, responsible for overall administration and holding the state seal [1]. The specific portfolio for foreign relations was entrusted to the Sumant (also spelled Samant or Sumanta), who acted as the foreign secretary handling diplomacy, treaties, and reception of foreign ambassadors; this assignment is explicitly noted in contemporary descriptions of the Ashta Pradhan [2]. Thus, while the Peshwa led general administration, the Sumant was the minister responsible for foreign affairs.

Sources

1. [1] History , class XI (Tamilnadu state board 2024 ed.) > Chapter 15: The Marathas > 15.5 Rule of the Peshwas (1713 - 1818) > p. 231
2. [2] https://www.britannica.com/topic/Ashta-Pradhan