Small-business groups are spearheading a legislative effort to undo a disparity in tax regulations that gives larger companies a much greater tax break for donating unused inventory, reports Bloomberg Businessweek.

C-corporations, organized under a structure typically favored by bigger firms, can take a tax deduction for up to twice the value of donated inventory or equipment. S-corporations, partnerships, and sole proprietors—called “pass-through” entities—can only claim the actual value of such items.

The different treatment stems from big firms’ “ability to twist the arms of legislators,” said Gene Fairbrother, a consultant to the National Association for the Self-Employed. That group and the S Corporation Association are lobbying for a bill before Congress that would equalize deductions for businesses in hopes of boosting giving by small firms.