Is the husband obligated to pay 3,000 pounds to his wife's sister, after she previously relinquished it under pressure and in need of money?

If your sister waived the amount owed by your husband with her consent, she cannot retract that waiver, because "what is relinquished does not return," as decided by the jurists. However, if she did not waive her right willingly, but was compelled to do so to recover some of her due, then she has the right to demand the amount, because consent is a condition for renunciation and donation.