backroom deal; under-the-table deal; secret deal
Refers to a deal or transaction conducted secretly, often with a negative connotation of being shady, illegal, or unethical.
It was reported that there was a backroom deal between the politician and the company.
彼は裏取引で大金を手に入れたらしい。
He seems to have made a lot of money through under-the-table deals.