A rendar is an official or landlord. Either way, a rendar is usually an authority figure.

Accordingly, a rendar can preside over an estate but may not own it. In ancient usage, he may lease the estate from a lord and collect rents on the lord's behalf. Today, a rendar could be a landlord or even innkeeper. He can make decisions on any issue connected to the piece of property that he presides over.

In the story, Gitl makes sure that all the wedding gifts for Shmuel and Fayge are piled onto two wagons. Among the gifts are crocks of butter, wooden bowls, and lengths of cloth. Apparently, the rendar himself sent a gift, albeit not a pretty one: a pair of silver candlesticks.

All of the main characters that Hannah meets in her mysterious world live in a shtetl. In Eastern Europe before World War II, most of the Jewish population lived in shtetls. In the story, the rendar would have presided over the land and shtetl. He would have been an authority figure in Shmuel, Fayge, and Gitl's world.

Source: The Classic Yiddish Stories of S. Y. Abramovitsh, Sholem Aleichem, and I. L. Peretz, edited by Ken Frieden (2004)

What a difficult question to answer! I searched Polish-American dictionaries and Yiddish-American dictionaries and couldn't find this word. I searched on Google and Yahoo and kept getting the message "did you mean render?" Finally, I decided to try a longshot and go right to the source: the author. I found Jane Yolen's official web site and sent her an email requesting a translation for this word. Ms. Yolen was gracious enough to write back to me and explain that a rendar was the head man of the village, comparable to a mayor in an American town. 

I hope this is helpful to you. Please visit Ms. Yolen's page and drop her a note of thanks if it is.