See S.A. C. M. 7, 1 A Ger can not be part of a Beth Din for Yisrael, even concerning money, but Ger can judge an other Ger. This is for forced din, but in non-forced din, a Ger can judge Yisrael (Sma sk 4, Shach sk 1). We learn this from the verse "מקרב אחיך" "from your brothers"
Source in Yebamot 102a:
A proselyte may, according to Pentateuchal law, sit in judgment on a fellow proselyte, for it is said in th Scriptures, Thou shalt in any wise set him king over thee, whom the Lord thy God shall choose; one from among thy brethren shalt thou set king over thee; only when set over thee is he required to be one from among thy brethren; when, however, he is to judge his fellow proselyte he may himself be a proselyte. If his mother was an Israelitish woman he may sit in judgment even on an Israelite. In respect of halizah, however, [no man is eligible as judge] unless both his father and his mother were Israelites for it is said, And his name shall be called in Israel.
There is a Machloket Rashi and Tosfot. According to Rashi, the Gemara rules only for Dine Nefashot (penal judgment), but for Money judgment, Ger can judge Yisrael. Tosfot does not agree. Rambam and Shulchan Aruch follows the same understanding than Tosfot (See also Tosfot 45b).