What is the origin of this word? There is a discussion about this in the gemara (Gittin 36b-37a):
מאי פרוסבול אמר רב חסדא פרוס בולי ובוטי בולי אלו עשירים דכתיב (ויקרא כו) ושברתי את גאון עוזכם ותני רב יוסף אלו בולאות שביהודה בוטי אלו העניים דכתיב (דברים טו) העבט תעביטנו אמר ליה רבא ללעוזא מאי פרוסבול א"ל פורסא דמילתא
"What is (the meaning of ) prosbul? R. Hisda says: Pros (an enactment of) buli and buti. Buli means the rich, as it is written, 'And I will break the pride of your power' (Vayikra 26:19). And R. Joseph explained: These are the bula'ot (city councils) in Judah. Buti means the poor, as it is written, 'You shalt surely lend him sufficient [ha'avet העבט is similar to buti בוטי] ' (Devarim 15:8). Raba asked a certain foreigner [who spoke Greek, Soncino suggests "linguist"] 'What is the meaning of prosbul?' He replied: 'The pursa (enactment) of the matter.'"
So we see here that while the Rabbis felt it was important to learn moral lessons by drashot that found Hebrew words in foreign terms, they also knew when to ask foreign linguists what the words actually meant.
Even now, we don't know exactly what prosbul meant in Greek. Klein writes:
Probably shortened from pros boule bouleuton ( = before the assembly of the councilors), from pros (= toward, to, against, before, in presence of), dative of boule (= counsel, deliberation, assembly) and general plural of bouleutes (= councilor), from bouleuein ( = to take counsel), from boule.The boule mentioned here is the same as in the gemara (although clearly the overall etymology is different.) The lower house of the modern Greek legislature is known as the Boule.
Steinsaltz (and others) also offer the Greek word prosbole, meaning the delivery, transfer in a sale.