The boss owes me a salary, wants my salary and gives me a receipt.

This is a very common thing. Many companies are like this now. Even the airport has to send a company to do this (collect money from my family privately). But the people at the airport intervened in the payment of wages and in turn charged me. So, never sign it. If there is any dispute after signing, it is a foregone conclusion. The other party will be malicious and will reply to you-call the police for a lawsuit.