If the husband and wife agree to write out the bedding for the house immediately upon the contract, is it valid?

The questioner : May Allah reward you. If the husband agrees with his wife to write out specific furniture, and before the furniture is delivered, he writes it out on a paper immediately upon the contract, writing out the house furnishings on a paper so that she is guaranteed her right?

The Shaykh : On a separate paper.

The questioner : Yes, before anything is delivered, immediately upon the contract, he names them...

The Shaykh : In any case, we have a principle, may Allah reward you, that whoever claims something that contradicts custom must provide proof.