Siman 21
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מי שקיבל עליו להשלים הדין ליום ידוע ונאנס. ובו סעיף אחד: מי שקנו מידו שאם לא יבא ביום פלו' וישבע יהיה חבירו נאמן בטענותיו ויטול כל מה שטען בלא שבועה או שאם לא יבא ביום פלו' וישבע ויטול אבד זכותו ואין לו כלום ויפטר חבירו ועבר היום ולא בא נתקיימו התנאים ואבד זכותו (מיד) (ואין נותנים לו זמן יותר) (ב"י בסי' י"ט בשם הרשב"א) ואם הביא ראיה שהיה אנוס באותו היום הרי זה פטור מקנין זה וישבע כשיתבענו חבירו כמו שהיה מקודם וכן כל כיוצא בזה (וכל זה כשקנו מידו אבל בלאו הכי לא דהוי אסמכתא) (מרדכי ריש סנהדרין ותשובת הרשב"א סי' תתצ"ט) וע' לקמן סי' ר"ז:
1 One with whom a formal agreement was made by means of a Kinyan that if he fails to appear [in Court] on a certain date and take an oath, [then] his fellow [-litigant] will be regarded as trustworthy in his pleas and will collect everything that he claims without an oath, or [he stipulates] that if he fails to appear on a certain date and take an oath and [thereby] collect [his claim], he forfeits his rights and will have nothing and his fellow [-litigant] will be exempt [from making payment], — and [then] the [stipulated Court] day passed and he failed to appear, — [the law is that] the conditions become effective and he forfeits his rights forthwith and they do not grant him an extension of time. However, if he produced proof that he was unavoidably prevented [to appear] on that date, he is free from [the obligation] of this formal agreement by Kinyan and he may [subsequently] take an oath when his fellow [-litigant] summons him again as in the first instance. And thus too, everything that is similar to this [case]. And this entire [ ruling is applicable only ] where a formal agreement was made with him by means of a Kinyan, but not otherwise, for [ then ] this would be a mere Asmakta.