Is it permissible to purchase with a salary guarantee voucher for direct payment from the bank?

If the transaction is valid, and the seller wants something to assure him of the client's ability to fulfill the installments, then there is no harm in him agreeing with the buyer to bring a salary slip and for him to collect his right directly from the bank. However, if the intention is for the salary to be a pledge (collateral) against the debt, then this is not permissible as a pledge; because it is a pledge for something that is not in existence at the time of the contract, and its existence or non-existence is uncertain.