In front of the cattle merchants; it is always מעשר, etc. - לפני סוחרי בהמה לעולם מעשר
Summary:
רש"י maintains that it is (an even) ספק whether the found money is from the buyer or the seller; we assume the stricter view and attribute it to the buyer. תוספות maintains that the majority of people in the vicinity of cattle merchants are buyers therefore the majority compels us to consider it מעות מעשר. (Otherwise it would not be an even ספק, for there is a minority of buyers who are using חולין money.)
Concerning לבער; No one argues (all agree) that it connotes the future - בלבער כולי עלמא לא פליגי דלהבא משמע
Summary:
The רשב"א maintains that the ברכה of להכניסו is said before the מילה, while the ר"ת maintains that it is recited after the מילה (because this ברכה is not limited to this specific מילה [alone], and עובר לעשייתן is required only when the performer of the מצוה recites the ברכה).
