It's a finance paper. Is there any problem with adding somebody with a Master in English as a coauthor, since he could help me correct the grammatical mistakes and make the paper more readable?
Co-authorship is based on contribution, not credentials. If someone participated significantly in the content of your paper, then they should be credited, even if they have an unrelated degree, or none at all.
However, editing is form, not content. A person who helps you with the mechanics of writing a paper, such as editing, formatting, transcribing, translation, etc., but not with the subject matter content of the paper can be given acknowledgement, but doesn't warrant co-authorship. General rule: if you can get someone from Fiverr to do it, it's probably not co-authorship.
It should be noted that there is a difference between editing and writing. If they wrote several paragraphs from scratch, found relevant references, and contributed conceptually to the presentation of the manuscript, in many fields that is an intellectual contribution that warrants authorship. However, if someone only edited your existing paragraphs, then this does not warrant authorship in pretty much any field of academia I am aware of. Payment for their services, or if they are a friend and don't want payment, a meal or gift is a nice gesture. If you don't pay for their services an acknowledgment in the paper is ethically obligatory in my opinion.