What is the ruling on money in excess of the device invoice if it is used for the same store, and what is the ruling on paying "al-khulu" (key money/premium) for commercial shops, and what is the ruling on Islamic banks in this situation where there is no partnership or trade contract but rather purchase by installments?

It is wrong to ask an Islamic bank to purchase equipment for a shop and then appoint you as their agent for the purchase, only for you to buy equipment with some of the money and other shop-related materials with the rest, subsequently submitting an incorrect invoice to the bank. You must inform the bank of what happened to correct the error. As for goodwill (commercial reputation), it is not money and cannot be sold. Regarding the bank appointing the ordering party as its agent to purchase and take possession of the commodity on its behalf, this is a matter of scholarly disagreement. The Hanbalis permitted it, while the Shafi'is prohibited it. The preponderant view is that it is impermissible for the bank to appoint its client as an agent for purchase in a Murabaha to the orderer transaction, in order to block the means to interest-based borrowing.