Is it permissible to buy a portion of an uncultivated agricultural land at the price of cultivated land and participate in its produce by way of "partnership," with the buyer knowing that his share has not yet been cultivated?

If the land is divided, and each person's share is allocated, then if the neighbors agree to gradually reclaim it and share in digging a well, planting seeds, and so on, and the produce is distributed among them according to the proportion of their ownership, this resembles a Sharikat al-Inan (partnership by capital and labor) and is permissible if there is no Jahalah (uncertainty).

If there is no agreement to reclaim all the land, then your colleague's participation with others in working and planting seeds to reclaim their land in exchange for a percentage of the produce is a permissible Muzara'ah (sharecropping) contract. This applies whether the seeds are provided by the worker, the landowner, or both. Muzara'ah is valid in all these cases, and the basis is whatever they agree upon, provided there is no Jahalah (uncertainty) or Gharar (excessive risk/deception).

There is no objection to your purchasing your colleague's share of the reclaimed land, whether it is a sale of an undivided common share or a specific, separated share. Participation in the produce is permissible until the reclamation is complete, and it is recommended to set a time limit for the reclamation to avoid disputes.