The heirs agree not to take anything of the cash left by their deceased mother, but to contribute it to Sadqah for Isaal-e-Thawaab for her.

Answer

It is not permissible. It is necessary to first divide the cash and distribute it to the heirs. Thereafter, whoever wishes to contribute for the Isaal-e-Thawaab, may do so. But this cannot be done prior to distribution of the assets. Also, it is not permissible to apply any pressure on any heir to make a contribution.