Modern Applications of Mi Shepara: Checks and Agreements

What about cancelling a check, or backing out of a memorandum of understanding?

R' Yitzchak Weiss, Minchat Yitzchak 5:119
[Checks are] like the case in Rama Choshen Mishpat 50:1, "One must pay to anyone who finds a document which states that X is obligated to Y or to any individual who finds the document."

R' Moshe Isserles, Code of Jewish Law Even haEzer 29:6
If a man gives a woman a document stating he owes her a dinar, that does not affect kiddushin.

R' David haLevi, Taz to Even haEzer 50:12
Today, the entire shidduch is effective by recording conditions. Therefore, even if they make a kinyan at the outset with witnesses and they record an outline, as is the custom, still, even if they do not say, "This is on condition that we will record conditions later," this is understood, and each party can still retract...