A pact is a formal agreement between two or more countries, individuals, or organizations. When pact refers to an agreement between countries, it is typically a written agreement. When we say that we have made a pact with friends, it can be any promise.
EX: After the war, many countries signed a pact, in which they agreed to help defend any member state if it were attacked.
EX: On the last day of the holiday, the three friends made a pact that they would all return the following summer and continue to be the best of friends.
The saying to make a pact with the devil, or also to make a deal with the devil, means that someone gets something they want in the present, but that they may regret it in the future. This expression originally referred to a person promising the devil his soul in exchange for a favor, but can now be used in the more general sense of making an agreement now that will bring trouble or harm in the future.
EX: If you make a pact with the devil, you have to be prepared to face the future consequences of your actions.
A deal is an agreement made between two or more people to take some sort of action or to establish a business relationship. It's especially common in business and political contexts.
EX:The tour guide has a deal with that hotel. Every time a tourist who he refers stays there, he gets a bonus of five per cent of whatever the tourist spends on accommodation.
We also use deal to talk about how a person or a group of people are treated. The expressions a raw deal and a bad deal mean that someone has been treated poorly, while we can square deal to talk about fair or honest treatment.
EX: Car workers are getting a raw deal with these new contracts. They need to pressure the union to negotiate better terms.