Seller not willing to provide receipt/ sign token agreement

Hello. We are trying to purchase an apartment and the broker has asked us to give the seller a cheque as a token amount. However, the broker says the seller is not willing to sign any token agreement or even provide a receipt. He says the cheque will not be depositted and will be handed back at the time of signing the Sale Agreement.

This seems very odd to me. Is this a common practice? Or should I insist on some receipt/ agreement for the token deposit?

Thanks