The contract should bein black-and-whiteto avoid misunderstanding. Here the underlined idiom refers to -

• The contract should be in black-and-white to avoid misunderstanding. Here the underlined idiom refers to - In writing.
In black and white
English Meaning: in writing or print / in terms of clearly defined opposing principles or issues.
Bangla Meaning: কোনো কিছু লিখিতভাবে পেশ করা

Ex. Sentence: I had to believe it, because it was there in black and white.
Bangla Meaning: আমাকে এটা বিশ্বাস করতে হয়েছে কারণ তা সেখানে লিখিত আকারে ছিলো।

Source: Website Lecture.