Can the bank fill in and cash a blank security cheque I signed for a loan?

A collections agent hinting they can write in any amount is describing misuse, not a legal right. A cheque taken as security for a loan is tied to the underlying agreement: if the bank completes it, the figure has to reflect the actual outstanding debt, and filling a signed blank cheque contrary to the understanding under which it was handed over can be challenged — and may itself expose the filler to liability. Bear in mind also that the UAE largely decriminalised cheques returned for insufficient funds; a bounced cheque today is primarily a fast civil enforcement route, with a partial-payment mechanism at the bank, not the jail threat collection agents like to evoke. None of this makes the loan disappear — the missed instalments are a real debt the bank can pursue properly. So take control of the real issue: get an official statement of the outstanding balance, approach the bank itself rather than the agent about restructuring or a payment plan, and keep every communication from the collections side, especially the threats. If a cheque is presented for an inflated figure, contest it immediately. A UAE-licensed lawyer can deal with the bank and challenge any misused cheque on your behalf.