45 C.F.R. § 2400.59
Fellows may accept grants from other foundations, institutions, corporations, or government agencies to support their graduate study or to replace any income foregone for study. However, the stipend paid by the Foundation for allowable costs indicated in § 2400.52 will be reduced to the extent these costs are paid from other sources, and in no case will fellowship funds be paid to Fellows to provide support in excess of their actual total costs of tuition, required fees, books, room, and board. The Foundation may also reduce a Fellow's stipend if the Fellow is remunerated for the costs of tuition under a research or teaching assistantship or a work-study program. In such a case, the Foundation will require information from a Fellow's university about the intended use of assistantship or work-study support before remitting fellowship payments.
45 C.F.R. §2400.59