Answer
(Fatwa: 1134/978=B-1429)
If, in the abovementioned condition, you purchase that land from your clients on their expected money, then you can handle it as you want. But, if you are waiting for another purchaser without buying from the clients and you are working as a broker in-between, then whatever is dealt between the client and the new-purchaser should be definite and known to the client, and also your wage should be fixed. It is not allowed to receive the extra money by selling with extra price and paying his expected amount without informing him:
وفي الدلال و السمسار يجب أجر المثل ۔۔۔۔۔ دفع ثوبا إليه وقال: بعته بعشرة, فهو بينی و بينك ۔۔۔ ولو باع باثني عشر أو أكثر فله أجر مثل عمله و عليه الفتوی (الھندیۃ: 4/450)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband