Is it permissible for an Islamic bank to stipulate to the purchaser that they guarantee the seller's performance bond, such that the purchaser bears any deficit incurred by the bank due to the seller's poor performance, and does this condition fall under the suspicion of inah (a type of usurious transaction)?

There is no objection to the bank or other party requesting from the instructing buyer a guarantee for the seller whom the buyer designates to the bank, so that the bank may purchase the commodity from him. It is permissible for the institution to obtain from the client a guarantee of the original seller's good performance of his obligations towards the institution in his personal capacity, not in his capacity as an instructing buyer, nor as an agent for the institution. This guarantee is given before the transaction takes place, and it serves to prevent any potential harm to the bank or other party. It does not entail that the guarantor becomes the owner of the commodity or an agent for its owner. The guarantee here is solely for good performance and fulfillment of obligations.