Section 37. A credit union may receive shares or deposits, under the terms of a written plan approved by the commissioner, from any labor union, association, trust, public utility, partnership, corporation, college, university, insurance company, charity, any city, town, county or district of the commonwealth or by any instrumentality thereof or individual for the sole purpose of making loans guaranteed in whole or in part by the Massachusetts Higher Education Assistance Corporation; provided, however, that no such plan shall discriminate against any applicant otherwise eligible for such a loan; and, provided further, that such deposits shall not exceed twenty percent of the assets of the credit union.