Shulchan Arukh, Even HaEzer 148

דין המגרש בגט ישן והמגרש בתנאי ושלא יתייחד עמה. ובו ב סעיפים:מי שכתב לגרש את אשתו ונמלך ולא גירשה ונתייחד עמה אחר שכתבו לא יגרשנה באותו הגט פעם אחרת כשירצה לגרשה ואם גירשה באותו הגט הישן ה"ז מגורשת ותנשא בו לכתחילה שהרי נכתב לשמה והרי נתן לה עתה בעדים כהלכתו ולמה לא יגרשנה בו לכתחילה גזירה שמא יאמרו גיטה קודם לבנה וי"א דהני מילי כשהגט ביד הבעל ומסרו מידו לידה אבל אם שלחו לה ע"י שליח ונתייחד עמה אחר שנתנו ליד השליח ונתנו לה השליח לא תנשא בו ואם נשאת לא תצא:

If a man wrote [a get (that is, a divorce document)] to his wife, and then he decided not to divorce her, and then he was secluded with her after he wrote it, he may not divorce her with that get another time if he decicdes to divorce her. And if he divorced her with this get yashan (that is, this get that was written before he was secluded with her), then she is divorced and may remarry, for this get was written for her and it was given with witnesses, as the law mandates. And why may he not divorce her with [with a get yashan] at the outset? [Because] maybe [she will become pregnant from the husband during that time of seclusion] and people will say that the get was delivered before she became pregnant. There are those that say that the aforementioned law [that if a get yashan was used, then she is permitted to remarry] only applies if the man delivered the get directly. However, if he sent it to her through an agent, and then he secluded himself with her after he gave the get to the agent and after the get was delivered, then she may not get married with the get [as evidence that she was legally divorced] but, if she did remarry [with it] she does not need to leave [the new husband].

כל המגרש על תנאי בין שאמר מעכשיו בין שאמר אם יהיה ואם לא יהיה הרי זה לא יתייחד עמה כל זמן שלא נתקיים התנאי אלא בפני עד אפי' עבד ואפי' שפחה (וביוצא ונכנס סגי) חוץ משפחתה או בנה הקטן מפני שאינה בושה לשמש בפניהם והדבר ידוע שאם נתייחד עמה בפני שני עדים כאחד וראה הוא והיא אותם אפי' נתקיים התנאי אח"כ ה"ז ספק מגורשת שמא בעלה וביטל הגט ויתבאר בסימן שאחר זה:

Anyone who divorces on condition, whether he says, "[You will be divorced] from now..." whether he says, "If it will be... and if it won't be..." he cannot be in seclusion with her as long as the condition has not been fulfilled, unless there is a witness present, even [if the witness is] a slave or a maidservant, and it is enough for [the witness] to come in an out [to be a random presence], the exception being one of her maidservants, or her small son, for she would not be embarrassed to have intercourse in their presence. And it is known that if he was in seclusion with her with two witnesses who were together, and they saw him and her, even if the conditions are fulfilled afterward, it is a doubt-divorce, for perhaps they had intercourse and the bill of divorce was made null and void, and it will be explained in the next siman.