setApprovalForAllof erc20 and er721 respectively.
depositit in the assetmanager contract in exchange for 1 million erc20 tokens. These tokens are deducted from erc20 owner account (in this case Alice).
withdrawmethod providing the token_id he is withdrawing. The assetmanager contract will deduct principal balance (1 million)+ interest from Bob's erc20 balance, deposit it in erc20 owner (Alice) and transfer erc721 token from Alice to Bob.