Is it obligatory to mention the commission amount to my partner in the land and divide it with him, or is it my exclusive share, given a prior agreement that any amount exceeding the agreed-upon price for the land sale belongs to whoever brings it?

It is permissible for two partners to agree that whoever sells the land for a price exceeding the agreed-upon price, the excess belongs to him, and this arrangement is akin to mudarabah (profit-sharing partnership). However, the partner must be informed of the true selling price; otherwise, deceit and dispute will occur.

If the commission is for brokerage, and the buyer knows you are a partner and gives you a commission, there is no harm in taking it, and it is not necessary to inform your partner. But if the buyer does not know you are a partner, then the commission involves deceit, and you must return the portion corresponding to your share of it.