Things by which a claim is valid but are unknown

The questioner:
He says: Honorable Shaykh, may Allah grant you success, you mentioned in your commentary on al-Zad a point that there are four things by which a claim is valid while they are unknown: the bequest, the acknowledgment, the compensation for khul' (divorce at the wife's request for compensation), and the mahr (bridal gift). I ask you to give examples for the last three.

The Shaykh:
In the sun.

The questioner:
The bequest and the acknowledgment.

The Shaykh:
We know the bequest; one bequeaths some money. Yes.

The questioner:
The acknowledgment.

The Shaykh:
In an acknowledgment, one admits to something, saying, "I have a right belonging to so-and-so; he has something with me." The acknowledgment is valid, and he explains it; he is the one who explains what this right is.

The questioner:
And the compensation for khul'.

The Shaykh:
The compensation for khul' is like when she asks for khul' for something without specifying it. It is explained as whatever is valid for it, whether it is money or something else, even if it is small.

The questioner:
And the mahr.

The Shaykh:
The mahr is known. The mahr is what is paid to the woman by the marriage contract. It is valid for it to be unknown, and he explains it as he wishes, because the explanation is left to him. Yes.