Commentary for Kiddushin 127:15

The mishnah seems to accord with Rabbi. A husband is believed to make a statement that would permit his wife to remarry without having to undergo yibbum or halitzah (either I have sons or I have no brothers). But he is not permitted to prohibit her, such that she would require yibbum or halitzah (I have no sons, or I have brothers). R. Nathan says he is believed in either case.