Sorry, but you can't 'offset' the benefits.
"WHAT TO DO WHEN A BENEFICIARY DIES:
A family member or other person responsible for the beneficiary's affairs should do the following:
- Promptly notify Social Security of the beneficiary's death by calling SSA toll-free at 1-800-772-1213.
- If monthly benefits were being paid via direct deposit, notify the bank or other financial institution of the beneficiary's death. Request that any funds received for the month of death and later be returned to Social Security as soon as possible.
- If benefits were being paid by check, DO NOT CASH any checks received for the month in which the beneficiary died or thereafter. Return the checks to Social Security as soon as possible."
Read the following:
http://www.ssa.gov/pubs/deathbenefits.htm