Dear Professor Koch,
I think the story of Viśvantara in the Tibetan translation of the Bhaiṣajyavastu of
the Mūlasarvāstivāda-vinaya is close to your example in spite of some minor differences.
It appears Derge 'Dul ba Kha232a-b:
de nas lha rnams kyi dbang po brgya byin gyis bus pa gzhon nu khyer ba'i bram ze ji ltar
thams cad kyi grong rdal du 'tshong bar brtsams pa de ltar rmongs par byas so//
de gnyis rgyal po'i zho shas 'tsho ba rnams kyis mthong nas de rnams rgyal po'i drung du dong ste smras pa/
bus pa nag po dang/ dra ba can ni bram ze zhig tshong 'dus na 'tshong zhing mchis (Kha232b) lags so//
des bsgo ba/ shes ldan dag deng la khrid de shog shig/ de gnyis khrid de 'ongs pa dang/ rgyal pos srang la
bcal te mnyam du gser gyis blus so//
Here the Brahmin sells Prince Viśvantara's son Kṛṣṇa (nag po) and daughter Jālinī (dra ba can), and the king,
Viśvantara's father, saves his grandchildren, paying gold according to the weight of the children.
This part is missing in the extant Sanskrit manuscript of the Bhaiṣajyavastu, and the Chinese translation and
a parallel story in the Saṅghabhedavastu does not say that the king had the children weighed and payed gold for them.
I hope this would be useful to you.
Sincerely,
Fumi Yao