1. Not every expenditure incurred in foreign currency nor amount paid to a person/firm abroad is liable to service tax under Section 66A. There has to be a service which is taxable and the foreign firm should not have a permanent establishment in India. In your case, please elaborate what is the nature of the activity pertaining to "Commission on guarantee" and whether any service is involved. If so, the taxability thereof can be examined.
2. Service tax in terms of Section 66A has to be paid in cash and cenvat credit cannot be used because such amount is not paid in the capacity of a service provider but as a service receiver under the legal fiction created by the statute.
3. If tax is payable under Section 66A, you can take credit of the same provided it is an input service for you in terms of Rule 2(l) of the CCR, 2004. In this regard, pl refer to CBEC Circular F.No.BI/4/2006-TRU dated 19th April, 2006 and Board's Letter F.No. 354/148/2009-TRU Dated 16-7-2009.