What is the ruling on a father allocating a plot of land he purchased to his male children only, to the exclusion of his daughters, given that the eldest son contributed a large sum to its purchase?

Justice among children in gifts is obligatory, and equality between male and female in them is obligatory, unless necessity dictates preferring some of them.

If you intend to transfer ownership of land to your male children, you must equalize between them and give the daughters similar land or compensate them with its value.

As for the eldest son's contribution to purchasing the land, if it was a donation, nothing is incumbent upon you. If it was a loan, you must repay him. If it was a partnership, he is entitled to his share commensurate with his contribution.