Learn Dutch: De fusie was een goede deal. - The merger was a good deal.

Because deal is an English loanword that Dutch has adopted as a de word. When an adjective comes between een and a de word, it gets the -e ending. If you are ever unsure, remember that the vast majority of English loanwords in Dutch automatically become de words.

How do you know that fusie is a de word?

A very reliable rule in Dutch is that nouns ending in -ie (usually borrowed from French or Latin) are almost always de words. You see this pattern constantly in words like de politie, de informatie, and de organisatie.

Are English loanwords like deal common in Dutch business contexts?

Extremely common. Dutch corporate culture heavily incorporates English terminology. It is pronounced just like it is in English, and its plural follows the standard rule by simply adding an -s to become deals.

How could I use a formal preposition like met betrekking tot (with regard to) from this lesson with this sentence?

You could place it at the beginning to set the context, which is perfect for formal business Dutch: Met betrekking tot de financiën was de fusie een goede deal. (With regard to the finances, the merger was a good deal.) Notice that starting the sentence with this prepositional phrase triggers inversion, swapping the subject and verb to was de fusie.