The husband wrote a receipt to his wife for the sum of £10,000, and the wife wrote a receipt to the husband for the sum of £10,000. In other words, each of them wrote a receipt, and the two receipts were deposited with a third party, on the condition that whichever of them transgressed against or mistreated the other party was proven guilty would be punished by the receipt becoming due to the one who was transgressed against, whether that involved reviling or slander. It was proven that the wife was guilty, so what should the third party do?.
Obliging the husband to pay money to his wife if he mistreats her or abuses her, or obliging the wife likewise, is not binding and in fact is not prescribed in sharia at all. Determining the degree of a punishment is not up to people, and imposing payment of money as a punishment -- even if it is permissible -- is up to the judge. Moreover writing a receipt to this effect is a kind of lying, because neither of them has received anything from the other in that regard.
Based on that, these receipts should be disposed of and should not be relied upon at all.
Each of the two spouses must treat the other well, and adhere to that which Allaah has commanded of kind and decent treatment. Allaah, may He be exalted, says (interpretation of the meaning):
“and live with them honourably”
“And they (women) have rights (over their husbands as regards living expenses) similar (to those of their husbands) over them (as regards obedience and respect) to what is reasonable, but men have a degree (of responsibility) over them”
And Allaah knows best.