Is it permissible to use international money transfer applications that may have varying exchange rates, and are the offers they provide halal or haram?

There is no objection to sending money through transfer applications for a fee, as a service charge. As for the offers:

1. The first free transfer is permissible.

2. Taking a sum of money for inviting a friend: If it is conditioned upon the marketer paying money, then it is a prohibited multi-level marketing scheme. It is permissible if the marketer does not pay money, and it is permissible if it is limited to inviting only one friend.

3. Additional money on the first transfer: It is permissible to take it if the customer's money is transferred directly and not held in the application. It is not permissible to take it if the money is placed in the customer's wallet within the application, because it is a loan from the customer to the application, and a gift on a loan is not permissible.