Yosef Karo, Shulchan Aruch — Choshen Mishpat 21
“ Paragraph 1- If someone made a kinyan that if he does not come by a certain date and take an oath, then his adversary would be believed on whatever he claims without taking an oath or if he made a kinyan that if he does not come by a certain date to swear and collect, then he has forfeited his rights and his adversary will be exempt and he will get nothing, and the day passed without him showing up, the condition has been fulfilled and he has forfeited his rights. ”
