Amud 20a

תוס' ד"ה שמע

We derive from this that Shmuel is correct - שמע מינה איתא לדשמואל

Summary:
One cannot be מוחל a חוב which was transferred במעמ"ש. There is no concern that the woman sold her כתובה במעמ"ש either because the עדים will verify the veracity of the claim or מעמ"ש is only by something which is עומד לגבות מיד. There is no concern for a מעמ"ש sale after גירושין for it does not pay. However by ערב or חייב מודה when there is an actual loss, we are concerned even if the swindler has to pay.

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תוס' ד"ה שובר

A receipt is effective from its date - שובר בזמנו טורף

Summary:
It is possible that even by איסור we say עבחז"ל since יש כח ביד חכמים לעקור דבר מן התורה. Two שטרות which have the same date, the one who received the שטר first is זוכה. However if they are dated on different days the one with the earlier date is זוכה even if he received it last.

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