They asked to confirm the income, although I do not understand how I will do it to them from Belarus.
Just use a friends KYC from outside Belarus and someone who can prove the source of income, someone who has savings or a stable job that is paid in via payslip or something, I had to do this in the UK for a bank transfer of £10,000.00, imo it is pretty normal for this to happen as its a large amount of money.
Anyway im sure your money is safe, and clarifying with them will release your funds, use a DEX next time.