Property business

I have a business of properties (such as real estate developer and property trading). In running it, I try to do transaction with minimum amount like recently a seller sell its property worth Rs 35 lac to me and i paid him only 5 lac and rest will be paid after 2 months. This deal is finalised. Now during this 2 months period i got a new buyer who is agreed to buy this property from me in 36 lac so in this way i get a profit of 1 lac. Is this profit halal for me. Similarly For example, mr. A has land worth 1 billion, and by an agreement I buy it and should settle the payment in a certain time period. During that time, I try to resell the land to get some profit, while the certificate is still in A’s hand

Is such transaction against the hadith which mentions that one shouldn’t sell things that are not in his possession?
If I based my practice on a hadith from ‘Aisha, which mentions that it is allowed to buy things with debt until a certain period of time, is it alright for me to buy and to pay it to it’s first owner?