Delay in Paying the Price
In case that the buyer does not pay the price on time, the seller has no right to demand an increase over and above the agreed price. However, assuming that the interests of the seller were damaged due to a diminishing purchasing power precipitated by the delay in paying the money by the buyer, there is a caution to make muṣālaḥah with the buyer on the difference in the price. 

To Pay to Revoke the Transaction
There is no problem in stipulating payment of (x) amount of money by the party who revokes the transaction within the binding contract or to conclude the transaction on such a basis. However, a period of time has to be set for the fulfillment of such conditions which have a bearing on the price of the sold property. Otherwise, it is invalid.

Deception in Transactions with non-Muslim
Lying and deception in transactions, even if the other party is non-Muslim, are not permissible.

Defect of Being Officially Non-transferable
If, after the transaction has been made, it is discovered that the property was officially not transferable which is deemed as a defect in the common view, it should automatically give the right of revocation to the buyer.

Selling on Credit    
In selling on credit, the period during which the settlement of the amount has to be made has to be determined. Therefore, if the sale was concluded without specifying a date for settling the installments, the sale is deemed null and void from the beginning. 
If the timetable for paying the installments has been agreed, but the buyer delayed the payment beyond the due date, this, per se, should not give rise to the vendor’s exercising the right to revoke.

To Pay Cash in a Salam Transaction
In a salam sell and buy, i.e. when the purchase is to be delivered later, one should pay cash at the time of transaction; otherwise, it is not valid.

To Sell an Apartment in a Salam Transaction
There is no problem in selling an apartment which does not exist in a salam transaction provided that the seller specifies its conditions so that there will be no principal ambiguty. At the specified time of delivery, if seller cannot deliver the apartment, the buyer may revoke the contract or wait until the buyer delivers it.

Buying and Selling Banknotes
There is no objection to selling banknotes for a price higher than their nominal value if it is concluded with a serious intention and for a rational purpose, for example, they are different in respect of being new brand/worn out, having special signs on them, or of different value. However, if it is not concluded with a serious intention but only to circumvent the law prohibiting ribā; it is ḥarām and invalid in Islamic law. 

Online Transactions 
It is permissible to conclude online transactions provided that the transaction is ḥalāl and its required conditions are observed.

To Sell a Check 
It is not permissible to sell a check to a third party for an amount that is lower than that written in the check. But it is permissible to sell it to the person who drew it.

Multi-level Marketing, Amway/GoldQuest
It is neither compatible with shar' nor permissble. It entails using ill-gotten property.

To Buy from a Company with Illicit Assets
General knowledge of the existence of illicit assets among that of a company should not cast a doubt on the validity of shopping in that company’s stores unless all the company’s assets could be dealt with by the mukallaf. 

Using ḥarām mixed Property
If there are both ḥarām and ḥalāl items, there is no problem in using some of them unless one of those used by you is ḥarām for sure.

To Buy Western Commidities
Should the buying of goods, which have been imported from non-Muslim countries, and using them contribute to strengthening the infidel and colonizing states which are the enemies of Islam and Muslims or provide them with financial support they may use to attack Muslims or Islamic lands all over the world, it is the duty of Muslims to refrain from buying and using such goods. 

Transaction with Zionist Companies
It is incumbent on every Muslim to refrain from buying, selling commodities imported from Israel. It is ḥarām to make use of goods produced by Zionist companies or companies which spend the profits to help the occupying state of Israel.