Is it permissible for the bank to take guarantees, such as the buyer depositing 20% of the asset's price with it, to prevent the buyer from backing out of the purchase, and what are the alternative Islamic guarantees if this form is not permissible?

There is no harm in the bank taking guarantees for the seriousness of the purchase order, as an expression of the buyer's earnestness in the promise to purchase and to avoid their backing out, which could harm the bank. It is permissible for the seller to take what they incurred from the earnest money margin if the buyer reneges, provided that they do not take more than the actual damage incurred, and they return the remainder to its owner.