You can avail a Bank Guarantee from the BNBL against cash or sufficient collateral as approved by RMA guidelines. 

Bank guarantees can also be issued against a counter guarantee issued by one of the financial institutions within Bhutan, or a bank outside Bhutan. In case a guarantee is to be issued against a counter guarantee of a bank outside of Bhutan, it must be routed from a correspondent bank of the BNB, and by means of an authenticated SWIFT message.  The underlying terms must also be discussed and agreed prior to sending the SWIFT message.