Notwithstanding the provisions of section three of this article, when by a special act of the Legislature any municipality or independent school district is authorized to become indebted for any purpose or purposes in a greater amount than is fixed by said section three, bonds may be issued under this article by such municipality or independent school district in an amount not exceeding that fixed by such special act. This article shall not affect any general or special law providing for the issuing of bonds for any improvement to be paid for in whole or in part by assessment against abutting property.